Well, there's a difference between what he/his companies may be worth and what he personally owns. According to Forbes, he's worth
around $3.1 Billion, but most of that is real estate that his company owns (which in turn means his father owned and/or was gained as a result of his father) and licensing deals (i.e., the "Trump" name, which, again, is his father's invention). His personal net worth is estimated by Forbes to be around $310 million, but it consists
almost entirely of inherited wealth as well).
But, yes, you're right, how can anyone say he earned anything at all when it was all given to him from birth?
And it's all been plummeting ever
since he started running, certainly, so, again, my guess is he's running again to try and make as much skim as he possibly can. That and ego, of course and I'm sure a huge dollop of what he thinks is momentum.
What choice does he really have? If he doesn't run again, he'll be seen as a quitter. He knows he has no real power, so he doesn't have to worry about actually doing anything presidential. He's already established that all he does is sleep, watch TV, tweet and golf. And occasionally he forces his sycophants to suck his cock on national TV. That's about it. It's the ultimate reality TV show for him.
Whatever meds he's on have likely destroyed his sex drive, so there's probably very little the Secret Service has to do or worry about. He's been an extremely useful idiot for the Republicans and the Russians, so they're all happy with him staying.
In short, he's a castrated fool on a hill, so why not?