why do people leave their league all the time when playing ladder?

Because of MMR decay.

they do it so they don't decay? or so they decay and get to vs people below their level?

if i leave my league before taking a 2 week break, i get to vs all the lil bronze nublets, instead of top silver?

Because when I go play a ranked ladder game I get placed into bronze due to the MMR decay.

So, I just easily leave it and never admit that I was ever placed in such a bad league.

ohh lmao, the people i see doing it are in plat/dia ~_~ guess dem leagues are too embarrassing

because they think leaving their leagues and playing a "placement" will increase their chances of a promotion (it won't) or they are embarrassed of a negative wlr which you can still see on their profile anyway

I did exaggerate but same concept.