Simple answer: They were not designed for cell phones.
Some of these games were designed before responsive (auto adjusting between computer and cell phone use) pages became the norm, and they became outdated, which is probably why they're available for free.
Some scripts have probably been updated, and the writer offered the original scripts to others. Computer codes can use input from keyboards and mice. A cell phone code uses different inputs.
Some may have been programming projects for courses, or even spare time practice.
Dave's Games was originally a Flash game site. When I started to have problems with some of the games, and found out that Flash will no longer be supported, I began looking for other options. What you see here was the best option.
First thing you should know: I am NOT a programmer.
I can tweak the look of many programs, but adding code to save scores is just a little outside my abilities. Saving scores the proper way involves using a database, and using PHP to send, retrieve, and alter database data. Basically, it means a total rewrite or heavy alterations to most of the games. The results then need to be displayed somewhere.
In order to keep high scores, the game needs to know who you are so it knows who got the score. That means I also need to incorporate the games into the system, which means you need to sign up for an account. Right now, you don't need to sign up. Just play.
With all that being said, I am hoping to learn how to do all that at some point. That's one of the reasons I had the Flash site before...High scores.
Personally, I find it easier on my eyes. It lowers the amount of light being emitted. Those of you who work in front of a computer all day know how tired and sore your eyes are at the end of the day. That's why you should also watch TV in a dark room. Too much light is not good for your eyes.
Black also uses very little energy (if any) to produce on a computer or cell phone monitor. Yes, your cell phone battery will last a little longer. (look it up)