Why is the artistry in old comic books so horrendously terrible, compared to their modern iterations.

Well what do you expect for $0.15 cents?
Now a days you need to spend $$$ for a book, and some of it trickles down to the art department.
R.I.P. Stan Lee.

Define “old” There was very good art work when the books were a dime.

There weren’t as many potential artists to pick from back in those days.

I agree, but I think it’s swung too far on the other side, modern day comic books look too glossy, and the colors really gaudy. Pertaining to superhero comics though, I’m sure indie comics look artistic.

Different times. Different artists. Different thinking. The art then was effective enough to illustrate stories and create a profitable enough industry which led to artists today still working on the same medium getting a chance to show their skills and earn.