Sphincter is a muscle. By learning to stretch it, you gain the capability to stretch/relax it on purpose, but it's not going to be loose by itself (unless you go too fast/too far and pull it); similarily how stretching skeletal muscles don't make your limbs weak and limp, it just gives you some extra mobility when you need it.
The whole tight/loose thing is more about control, and the willingness to apply it. Like, most people who are not anal fanatics (on the receiving end) remember and repeat the one thing that is mentioned in every sex publication/blog/instructional/forum: "the key to anal is TO RELAX". And that's all they do. For a woman, it (usually) feels good and even desired to tighten the vaginal muscles (a la doing Kegels) during an intercourse for a tighter fit. Doing the same with the anus seems counter-intuitive, as every fucking guide says "relax as much as you can". And so they do.
Plus (reportedly), most women who are into anal enjoy vaginal wall (a-spot, g-spot) stimulation via rectum and not the anus stretching itself, so tightening the sphincter during anal intercourse won't do anything for themselves, it only makes difference for their partner (similarly, for men who receive anal it's usually first about the prostate, not anus, stimulation).