Welcome to Biology.SE!
There are a many misunderstanding of what evolutionary processes are in your post and it is therefore quite hard to answer (hence the current two close votes I guess). You might want to read some introductory textbook on evolutionary biology or other online resources such as the "understanding evolution" project.
The most obvious misunderstanding is that evolution is NOT some kind of forever "improvement" process in a static environment. Evolution is something else! But I can't make a good overview of evolutionary processes in just one post.
[why don't we live on] average of 300 years?
You may want to read about this post concerning the evolution of aging but it would probably be slightly too advanced but you would probably still get something out of the answers.
Why don't we just keep evolving [..]?
Humans, definitely keep evolving. Nothing have stopped. We evolved as we have always and it won't stop until we get extinct! This post may for example help you to understand that evolution keep occurring today in humans.
When you say
Humans (homo sapiens) have been on the earth for thousands of generations it sounds a bit like human have came to existence some time ago and have evolved since. It is a terribly misleading point of view, one of the reason is that one cannot date the existence of the first humans without making a very arbitrary decision about what a human is. A more correct way of looking at life on earth is that everything have come to existence about 3.5 billions years ago and everything have kept evolving since. Any single living thing on earth have been evolving for the exact same amount of time. Humans have evolved for 3.5 billions (even if we don't call earlier forms as humans) and unicellular algea have evolved for 3.5 billions years (even if we don't call earlier forms as algea).
Hope that helped!