You clearly never played an SF game in your life.
SF gets a significant change with each title. The number doesn't change because there are no significant core gameplay changes, it just adds to the previous title.
For instances, SFII to SFIII (Yes, I'm aware there was Alpha/Zero in between... We do not talk about it) changed a lot. The cast changed entirely, the game engine changed to the far superior CPS III engine which allowed bilateral 2D sprites (instead of mirroring them), it introduced a number of different mechanics that didn't appear in SFII (parry system, no blocking in the air, damage was completely reworks, you can select supers...etc).
SFIII itself has 3 iterations: New Generation, Second Impact and Third Strike. These don't change the core dynamics and mechanics, but simply improve upon them. Balance is tweaked, new characters and content are introduced, graphics are "cleaned up", maybe tweaked a bit too, like the HUD... etc. But the base is the same.
Then we got SFIV. SFIV, again, is a complete rework from SFIII. The game engine is completely different, being rendered completely in 3D (even if it does play in 2D), it does away with the parry system in lieu of the Force Attack mechanic, the story and cast are completely different again, being "located" time-line wise between SFII and SFIII, reintroducing a lot of cast members, the mechanics change, it's the first street fighter (IIRC) with online multiplayer...etc.
SFIV, however, is getting it's own iterations: SFIV (dubbed Vanilla), Super Street Fighter IV (dubbed Chocolate... don't ask), and now Arcade Edition. It's always the same game, it's just constantly tweaked and added onto.
SSFIV adds a LOT of cast members to SFIV, it changes the PP/BP online system, it received a lot of tweaks to the online in general, balance changes, etc. One could argue whether the balance is better in Arcade Edition or SSFIV, but AE brought 4 new characters, a MASSIVE and much appreciated revamp of the replay system, a few extra tweaks, and was only priced at 15 bucks. PC users (like myself) got a particularly huge boost from it, since we never got SSFIV, so we got AE + SSFIV at roughly 30 bucks.
Also we gotta keep in mind SF is the de facto leader and benchmark of all fighting games, and has been for a few decades for a good reason.
It ain't perfect, but compared to CoD and Madden junk, I'd say it's far more worthy of my money.