Because their products are not enterprise-level. I work in a company that provides macbooks to developers and we have over 500 apple silicon macs and managing them is an absolute nightmare.
Hardware is terrible quality, they have so many issues with faulty screens and charging and every time you have to send one back to Apple via a reseller it takes weeks and the fix cost is usually not worth it, so we have to write them off before they depreciate. Intel Macs often have issues triggering the installation for some reason, sometimes it takes dozen reboots to kick it off.
There's no staging for the OS, each device gets updates straight from the internet and every update breaks something. There has not been a single minor update in 13.x and 14.x that hasn't broken some backwards compatibility (breaking Keychain seems to be Apple's favorite thing) or changing user's settings such as notifications or security&privacy breaking third party software annoying the user and making our lives miserable and there's nowhere you can raise an issue, so you can't rely on building out (limited) automation because API calls keep getting removed, changed or removed in binaries but left in code like the call to trust a cert key in Keychain...
Safari is absolute trash. I have the worst opinion about people who made it, that browser doesn't not belong anywhere outside of testing. It's a complete and utter garbage, the only browser that doesn't know how to handle SSO sessions, can't remember certificate preferences and keeps prompting user for authentication when accessing Keychain when other browsers don't.
Apple Mail is the next worst piece of software I've ever whitnessed in my entire life, it's possibly the only mail client in the entire world that doesn't know how to re-use a connection to an IMAP server, it opens a new connection for every single thing possible absolutely hammering a mail server. This is just absolutely insane.
MDM is incomplete because of MacOS restrictions, there are lots of things you can't do which you would expect from the most basic MDM tool and the most annoying thing about it is that signing into an iCloud account prevents MDM from wiping the device essentially giving someone a device free of charge so what's the point of MDM then...
Apple's devices are built for retail consumers, not enterprise users.