Creating an AD domain carries a substantial amount of extra overhead that they might not want to deal with. The basics of setting one up are simple enough but actually building out/maintaining the infrastructure the correct way can be a lot of extra work (2 DCs for redundancy, sites configuration, users, groups, initial GPOs). There are also licensing and CAL considerations (bare metal and hypervisor, both different), domain and forest options that can paint you into a nasty corner of you’re not careful, and a whole host of other things to think about and plan around. I’m not arguing that a domain is bad, on the whole I agree 100%. I just like to set the record straight that building a new production domain isn’t as simple as a lot of people would have you believe, and OP might not have the time to go through all that.