Because constructive criticism makes the world go around. There are things I like about iOS, that’s definitely not one.
There are also ethical issues involving the lack of sideloading. Imagine Microsoft had never allowed people to install unapproved software in Windows. There would be a lot of Chinese folks that get around the great firewall that never would’ve been able to do so. Taking sideloading away makes it very easy for the CCP to just say “don’t sell that on your store” and for that to be the end of the story.