no one is saying that terran will never win against zerg muta/ling, but the odds are heavily stacked against them.
tanks take too long to mobilize. thors, does anyone even go for those? sure they're a decent (at best) muta counter but mass lings will own them. plus they're expensive as hell.
stim, you can only run away for so long on limited time and the lings will catch you eventually. the pros do stim to run away and banelings still catch up eventually. you will see them split up their forces to minimize casualties and they still end up losing the majority of their units.
PF gets destroyed by banelings because they do so much damage instantly. bunch up 15 banelings and it's like a nuclear launch. you can't repair it in time, and you can't repair it anyway since the scv's will die from AOE damage.
repair, really? who is going to repair tanks/thors in the middle of a battle? you risk losing scv's from baneling AOE.
just look at the replay of these top sc2 players and see how hard it is for terran to stop zerg.
http://www.youtube.com/user/HDstarcraft?blend=2&ob=1#p/u/21/kFkvyQaqJ44
http://www.youtube.com/user/HDstarcraft?blend=2&ob=1#p/u/32/E6cqfgQ6j34