Because companies who make Antivirus/AntiMalware software good LOOKING for samples themselves. That don't need to use the very lazy and ethically iffy method of using the general public as test subjects.
Because it's an irritating piece of garbage-ware that gets in the way more than it "protects" and even when disabled still gets in the way because the associated services don't stop running. When I disable something I expect it to actually stop running. This is akin to Microsoft installing a piece of behind-the-back-ware, along with all of the rest of the behind-the-back-ware, that is installed with every default install of Windows.. You want to use it and let that crap control your system and many aspects of your computing experience, go right ahead.
I have monitored my system to see what happens when its disabled. It doesnt do anything, the service runs, but it doesnt actively do anything. I compared it to using a 3rd party tool to "properly" disable it where that actually kills the service but there was no resource impact or change of behaviour in related to things like uploads to microsoft servers or scanning of system activity, the only difference was the latter lost the ability to do scans on demand.
You can also control defender like the advanced malware products using group policy, cloud behaviour etc, is just the controls are not made as easily accessible.
I can understand why people want it off, although I think removing it from the system is overkill and can potentially break the OS like when people had issues with the update in May because they had removed Edge from windows. Defender in its default mode with behaviour blocking on and real time file scanning on is quite heavy on the system.
It is always an interesting topic, as is performance vs security vs convenience. you give the people who do the silly things to get malware the power to easily bypass the protection then they will let the malware when run when something like norton offers you the chance to let it run, so can understand why Microsoft doesnt make it as easy to bypass defender's decisions.
In terms of data hoarding, today I spent some hours cleaning out google's password manager, I enabled it on a phone to make it easier to login to two apps, then later found it auto added behind my back 400 saved passwords from an old chrome desktop install.