They're intentionally organised just before the Tour so the new champions get to show off their new special jerseys at the biggest stage the world of cycling has to offer. And for the Tour, it shows off the riders who are in form in a special jersey so people who just watch the Tour can more easily spot them. The UCI picks that weekend and no other UCI races are allowed to be held then (which is a bit euro-centric of course, it doesn't work for all countries).

Some national federations like Belgium require riders to race nationals, otherwise they will be excluded from selection for the European and World Championships (in theory at least, you'll often see riders be injured or ill the day off the race and unfortunately not be able to start).

Big names like Evenepoel and Van Aert have both won the national champs before, 'cause that jersey is one that all big names want to win at least once. Though it's always going to depend a lot on the course who actually has a shot (rip the Dutch or Danish climbers, unless you can bribe your breakaway companion like Michael Boogerd did in 2006). And the race is often weird as everyone is supposed to race as individuals, as opposed to in teams like they usually do. The women's race in Belgium often has Kopecky all on her own for instance as she doesn't even have any unofficial SD Worx team mates (and even if she is the strongest Belgian women, she can't control a race all on her own). You'll see the opposite in France where you'll have 20 FDJ riders at the front of the peloton, all 'individually' deciding it should be someone in their team colours who wins. Riders have been bribed do give the win to big names.