all nice explanations the other commenters gave you, but the real answer is that it stems from the Demotivational Posters ( https://knowyourmeme.com/memes/demotivational-posters).
some people thought it was funny to take bland motivational quotes on a spin. many people liked these and just took the template and used it for any kind of memes they thought of.

for example: normally these posters have black frames (like you mentioned), an image in them, a title underneath and then a short summary or explanation.
here you can clearly see that it has a title, despite the explanation is the same. normally you wouldn't want to repeat your joke, on demotivational posters you do

the monitors were smaller then, and when you time travel with stolen old memes and put them on new screens...its a resolution thing..ask a scientist i dont know man im stupid.

Dark themes on websites are more recent, users wanted to introduce more dark so as to rest their tired memeing eyes

Probably cause it's easier to add text this way