Also knowing what security risks are in play with N year old OS lets them decide if they want to allow the user to proceed or work around. Some applications are more aggressive in what version OS they need and that's a driver for upgrading phones, pushing the OEM to keep a phone in support for longer, or third party ROMs if this catch22 wasn't part of it.
I've seen plenty of apps need a minimum major version of Android, but never any that have needed a minimum security patch level.
There's simply no universe where an employee at a typical F500 company is going to be able to convince the IT team that they should permit custom code on phones even if the logic of the argument is sound. Even if they found the person, persuaded them, got them to persuade their management, in house legal teams, compliance teams etc., I seriously doubt whether the MDM solutions (eg InTune) would even have a checkbox to override the policy on one phone. To say nothing of the ongoing cost of tracking that one employee's specific setting.
This also plays a bit into the argument of why integrating your personal device into the corpo ecosystem of your employer is not a decision to be taken lightly.
That's my point: I know that's how it is today, but the reasons for it being that way are illogical.
The same applies to most open source efforts, but I think it's understandable for institutions with consequences to what their apps do (like handling money and bank accounts) to opt out of potentially being undermined by the OS when they have the means. There's also elements of phones being general purpose computers now vs locked down appliances, GPL3 vs tivoization, and so on.