Two main reasons.
I've seen a lot of comments on here about putting in the work. It really is exhausting. Especially when so many couples make it look so easy. I've put my heart, mind, spirit and soul into serious relationships in my adult life and am frankly exhausted of having to hit the reset button. It sucks going from deep connected conversation to "what's your favorite movie?"
My most recent relationships ended strangely and poorly, which doesn't exactly keep me amped about meeting new people. One is actually still stalking and harassing me while she continues her self destructive spiral of psychotic life choices. We knew each other for 3 months and she's been harassing me for a year since we broke up. I don't want to drag anyone through the insane shit I've had to deal with.