So that when a teenager types "fortnite" in on their phone they can install it. That's the only reason. It's a little different from organic "discovery", like search and recommendations.

FWIW, Google (illegally? according to Epic, anyway) blocked Epic's attempts to address this through partner deals with phone manufacturers, so their hand was relatively forced.

If Google and Apple didn't require all installs to go through a blessed path, people would just type "fortnite" into their search engine of choice and install that way instead. Of course, Google & Apple have good reason for not allowing that, but in the case of Fortnite they really are not delivering much for the 30%.