1. Fox needs to get close to us in order to hit us with utilt. Unlike Falco if he messes up it costs him a stock. Utilt also has very limited range. If ftilt is easy to avoid for Fox than utilt is exponentially easier for Sheik to avoid. I'm also fairly sure that utilt chains is escapable.It's easy to list rando mthings fox can do in a match up that falco can't but instead I'll just point out these facts.
1. fox has 40-60% damage true combos on us too... utilt->utilt->utilt->usmash
2. falco does die to ftilt though, gotta learn to stall your ftilts to get the maximum number of tilts in while still comboing. Not very difficult.
yeah the match ups are different, but if ftilt is the only reason sheik beats fox... then she should be winning versus falco just as often. there are tons of ways for fox to deal damage and apply pressure without much fear of ftilit locks, and teh window in which you can ftilt lock doesn't consume the whole game anyway. against good foxes (or maybe more acurately) ones who know the match up, ftilt may net you one stock.
2. Falco doesn't die from a single ftilt lock. You hold up and DI the usmash correctly and he is just too heavy.
How does Fox deal damage and apply pressure without having to fear ftilt? He can't damage us from long range due to crouch. because of that he has to get close to do any damage at all and ftilt will flat out beat or outrange the majority of his moveset.
Also the window for ftilt on Fox is pretty large when you consider the fact that Fox is extremely light. So half of his stock is pretty much in the window.
That is even more evident when you simply choose not to hit him with anything else once he's in the window.