Why does Joshua and Wilder have problems with Fat Boxers?

Because heavyweight boxing is not the 'physique' division or the 'appearance' division but the 'HEAVYWEIGHT' division. Weight, contrary to popular belief, is a huge advantage in the heavyweight division and it doesn't even matter if the weight comes in the form of muscle mass or fat.

The name itself, 'HEAVYWEIGHT' tells us all we need to know.

Provided a heavyweight is properly trained and conditioned, the heavier heavyweight is going to have the advantage against a similarly trained and conditioned heavyweight who is lighter.

You can apply this very same thing to the Klitschkos too. Klitschkos had some of their toughest fights against chubby / fat heavyweights (Samuel Peter, Corrie Sanders, Ross Purity, Lamon Brewster) and not heavyweights that had a bodybuilder like physique (like David Haye).

In the heavyweight division, fat / additional weight is an advantage. Just like it is in power lifting, strongman competitions, shot-put events, sumo wrestling and so forth so on.

Heck, the greatest MMA fighter in history (Fedor Emelianenko) never had a ripped physique with visible six packs like a bodybuilder.

So ridiculing a heavyweight due to his physique ALONE, is one of the most ignorant and superficial things a person can do as a boxing fan.

The idea that fat = disadvantage at heavyweight or muscles / less weight > fat / more weight at heavyweight is an unproven speculation / assumption to begin with, based on no concrete evidence.

People are too brainwashed by HOLLYWOOD and propaganda!