In what way is it time consuiming? It takes just as long to buy an AMD based PC as an Intel one, ditto installing Windows, apps, etc. Other than creating a new master image with the required drivers I don't see what is different and you would need to do that for a new range of Intel based PCs anyway.
It's a little more complex than that at a large enterprise.
New hardware means new drivers as you pointed out, which is for all intents and purposes a new O/S from a SOE (Standard Operating Environment) point of view. You have to build a new SOE (master O/S image with drivers and other required software). As part of building that new SOE, all the apps used by the enterprise will have to be certified for the new SOE - will third-party vendors support that software on a non-intel PC - that's not as silly as it sounds in the Enterprise, paid support space. They also have to support the new SOE. There will be different support processes (i.e. different error messages that will need new or updated support flowcharts, documentation, wiki entries, etc. for the non-technical frontline helpdesky people). New hardware to support, so instead of having 30k desktop of poduct X that all use the same BIOS/UEFI firmware images, drivers, installation routines, patches, etc., now they could have 20k product X and 10k product Y that also has to have the same effort put in (i.e. rather than effort X, it's now effort X + effort Y).
You are right in saying it is no different when doing a major desktop replacement, going between generations, where you'll have a desktop replacement program and for a year or two you'd have to support both as the migration progresses. But those are usually planned and budgeted process. The issue here is that this is an unplanned, un-budgeted situation. This could be happening mid-cycle, i.e. they expected (and budgeted for) only be supporting X, but because of the chip shortages they can't get any more (or enough) system X's for their current needs, so if they want to have enough desktops, they have to get system Y at a time in their budgeting cycle they hadn't budgeted to have to support two systems, so they might have to support 25k X's and 5k Y's, each of which take the same SOE-building and certification effort.
Or, even worse, they already were in the middle of a desktop replacement cycle, having to support (Intel) X and Y systems, but they are unable to get enough Y's to finish the migration, or enough X's to offset the Y deficit, therefore they now have to get (AMD) Z's to create a 3rd SOE and use Z's to complete the replacement of their outdated X systems. So now during their aborted Y migration, where they've had to substitute unfilled Y orders for Z's, they might have to be support 10k unreplaced X's which are being replaced by Z's, 15k Y's which they can't get any more of, and 5k Z's (whilch will eventually be 15k once all remaining X's have been replaced with Z's). So they've had to build an additional SOE on top of what they budgeted ($100k+, possibly way more) plus support three, rather than 2, SOEs during this transition. Which means instead of supporting just one SOE/firmware system for the 3 years between cycles when the replacement is completed to the beginning of the next replacement cycle, i.e. just Y, they now have to support Y and Z for the rest of the 3-year cycle until they can do a full refresh of only 1 type of platform - during which time they'd again have to support 3 SOEs until they'd done a refresh of Y+Z with a single new type of system.
When you are talking large corporations, thousands, tens of thousands, hundreds of thousands of desktops, it's a bit different then just going down to the local computer store, ordering 20 computers to pickup next week, and replacing all 20 of your business computers in one weekend with a single office IT nerd instead of a dedicated IT team doing all the work ...