Why are my legs so tight?

So I stretch almost daily and before each workout. When I’m stretched out I can put my knuckles on the ground in a hamstring stretch. But my legs tighten considerably during the day and a lot at night (to the point where I can barely reach mid shin in a hamstring stretch) and it takes me a few minutes to limber up.

I drink nothing but water all day aside from coffee in the morning and some drinks on the weekend.

Normally, this is just frustrating and leads to a little stiffness in the am which goes away once I’ve started moving around for a while. But lately it’s been resulting in pain in the lower back that has really limited my workouts. Still gets a little better through the day as I move more, but movement is very limited whenever I’ve been sedentary for any appreciable amount of time (have a desk job but try to move regularly). Same thing happened a few years ago and I got dry needling which really helped with some additional stretching. I guess I’ll be going back.

But what I’m really interested in is the “why” and whether there’s anything I can do in addition to religious stretching for preventative maintenance. Anybody else have this issue and what have you done to address?

TIA