It is about security for them. One of the big problems of sideloading are the idiot users. Apple has a good name for iOS security because it is a walled garden. Everyone expects shit on android but never does on iOS, which is due to good marketing on apples side and the walled garden. Now your idiot user installs an app from weoffervirusesandcryptominers.com and like always it will be apples fault that something went wrong. Customers are idiots and for a company that sellers perceived security and lifestyle products free reign for idiots is a serious business risk.