My thoughts on why he’s not doing it, could be an any/all situation:

a. there’s no way for him (or his family) to profit off of it
b. he really does think this is all a hoax
c. he doesn’t care because he’s (allegedly) not sick and/or if he does get sick he knows he’ll get treatment because he’s POTUS, so fuck everyone else
d. someone he doesn’t like told him he should do it, so he’s not doing it out of spite
e. someone he likes (and/or the last person he talked to) said he shouldn’t do it because reasons, so he’s not doing it
f. he wants this to be bad so he can argue that we have to shut down immigration and global trade completely in order to make America a third world country great again

If it was only one reason and not some combination, I’d say (a) is like even money; (b) is probably like 3:2 odds, (c) is probably like 7:2, (d) is 5:4 odds, (e) is 4:1, and (f) is maybe 9:2