The reason why that keeps getting said is because, ultimately, that’s the main reason why Ryu wins. Obviously, it’s not the only reason why Ryu wins, but it’s the primary reason.
Let’s take the match-up from Ken’s perspective, since it’s one I’m more familiar with. In my experience, Ken has a slight advantage in this fight, also for the same primary reason: He can out-stone Guile. Ryu’s FBs are faster, so he has an even easier time, plus his HK is much better. BUt I could be wrong. Anyway, here’s how the fight goes with Ken:
As soon as the fight starts, Ken does one of two things: Walk backwards or jump backwards (optional air HK). The reason for this is that Ken’s optimal range in this fight is between 1/3rd and 3/4ths of the screen away. From this range, Guile can’t do anything except pray to hop a FB and nail Ken with an early j.forward/roundhouse if he’s lucky. Otherwise, Ken stones away, looking to DP Guile if he jumps and sticks out anything (which is much more likely to happen).
UNless you happen to be some sort of charging machine (I tried the charge trick and couldn’t get it to work), Guile can NOT keep up with even Ken’s assault. At some point, he’s forced to jump up/over a FB or block it. This doesn’t sound like a big deal, but Guile’s main problem is causing consistent damage in this fight without taking much damage himself. It’s much much easier to Ken to whittle Guile down than the other way around. Say Guile jumps up over every FB - Ken doesn’t care, he just continues to stone away. If Guile jumps up, walks forward, jumps up, then Ken has to do something. What he does depends on what Guile does. If Guile walks forward, Ken jumps at him. Ken does jumping fierce. This move seems to have a stupidly huge hit box on it. Only Ken seems to have this, not Ryu. Ken’s jumping fierce can beat Guile’s c.fierce clean. Even if it hits high, it stuns for pretty long (like many shoto attacks), allowing Ken to land and mash on c.short. Guile can try and sack throw when Ken lands, but it’s random. More than likely Ken lands and gets to land c.shorts. He gets in 3 or 4 c.shorts and finishes c.roundhouse -> FB to push himself to safety. NOw Guile is near dizzy, and if he eats another attack or two, he’s dizzied, and there goes the round.
Guile can anticipate Ken’s jump and try to air-throw him, but it’s a guesing game in Ken’s favour. Ken can do an early j.fierce if he sees Guile jump, or do j.fierce -> air HK. If the j.fierce misses, the air HK has a good chance of knocking Guile down, and Guile isn’t going to be i nthe dog house for at least the next 10-20 seconds.
If Guile manages to get in close, he has two main options: c.forward to snuff FBs attempts, or Sonic Boom to match FBs, followed by backhand. In my experience, c.forward hit trade is not in Guile’s favour, because after two trades Guile gets knocked down (possible dizzy; round is over), and the damage isn’t that great to risk that loss in positioning. As for SB + FB = backhand, Ken doesn’t really care, as it will happen maybe once in a match, and the damage is minimal to Ken. After one backhand, Guile is too far away to do it again.
But anyway, if Guile is in close, Ken does one of two things, depending on what Guile is doing. If Guile is standing, it means he’s not charged for a FK, so Ken jumps and has a great chance of nailing him with his j.fierce. If Guile is ducking, Ken backs off a little and reassumes his optimal range. The main risk that Ken runs constantly going back is that he can eventually get himself cornered, so at some point Ken will want to move forward. If Ken is cornered, Guile does have a chance to keep him there, but Ken has a much better chance of getting out (DPing a limp for example, or HKing over a Sonic Boom) than Guile does if he’s cornered. And Ken has a much better chance of cornering Guile simply because Ken, at times, has free offense - Ken can jump in on Guile as long as Guile isn’t crouching, whereas Guile can never advance/jump on Ken unless Ken throws a FB at the wrong range, which won’t happen against a good Ken, and even if Ken does throw a FB at the wrong time, the penalty is usually a jumping forward/fierce/roundhouse (combo is highly unlikely, since Ken would have to pretty much throw a FB after Guile jumps in order for Guile to land a deep enough move to combo; the best Guile can hope for is jump attack, c.forward, Sonic Boom to mount pressure, but Ken has ways around this). Once cornered, it becomes even more of a guessing game in Ken’s favour. Ken can’t keep him there indefinitely, but Guile is going to lose a chunk of life before he gets out of that corner.
When you combine the life that Guile loses when in the corner with the life that he loses out in the open, and the fact that Guile has less chance to do consistent damage unless he manages to push Ken in a corner, which Ken can do more easily, plus the fact that Guile has practically no come back power in this fight, that’s when it becomes clear that Ken has the advantage in this fight. Of course, this isn’t exactly an easy fight for Ken - he has to be constantly aware of his position in relation to Guile’s, and when to throw FBs and when to jump forward/up/backwards, but I think it’s in his favour. The primary reason is because, out in the open, Ken can control the match more easily by, ultimately, throwing a ton of FBs at Guile.
Now, if Ken can do this, I don’t see why Ryu can’t. Ryu doesn’t have Ken’s j.fierce, so he can’t jump in on Guile as easily, but Ryu can out-stone Guile out in the open far more easily than Ken. Also, Ryu’s one hit knockdown HK, that does huge damage and has a frame of invinciibility on start up, gives him options that Ken doesn’t in close. BUt yeah: “Guile loses, cause Ryu can throw FBs faster” is the short way of analysing the match.