Why do good runners feel the need to do marathons when clearly past their peak?

Worked for me.

Because I enjoy them. I don't think I was ever really good, but I ran in college and used to contend for overall wins in local races. Now I'm old and slow. I run marathons because I like running them. I'm a hobby jogger now (really always was), so I do what I like. What's it to you?

Here's the thing, no one cares what you think

$$, dummy

^^^

These kinds of posts always confuse me. Why would people who have enjoyed challenging themselves and who have enjoyed competing suddenly no longer desire that?

Maybe the real question that you want to ask is why they don't run marathons sooner, not why they run them at all. But you have kind of answered that question, at least in part: it is a demanding race to train for, and many people in their 20s don't want to undertake the training required to do them. Also, it is awfully hard to move back down in distance after moving up (I cannot explain why, but trust me, it is), so why make the jump so early?

I was a good but not great college runner. I came back to the sport after a fair amount of time away. I knew I was never going to come anywhere near my college times, so I decided to see what the marathon was about, even though I was in my 40s and well past my prime.

The journey of training for something new was in and of itself a reward. In addition, I got to find a new test for myself, one that wouldn't pale in comparison to my past efforts since I had no past efforts at the distance. I got to compete against others and against myself. I got to experience a totally new aspect of a sport that I had already been involved with for almost 25 years.

It was all a great experience.

Because marathon is life.

I don't know what you mean by good runners. If you're really good you can make a lot of money running marathons even if you aren't quite as fast as you once were. That aside, I don't know why someone who is a serious runner wouldn't want to see what sort of marathon they can run. It's a classic distance and an intimidating one at that. I never found anything in the sport as satisfying as running a good marathon was whatever "good" may have been at the time.