Dialup over Ethernet

A client had a (business) customer switch over to UFB and needed 
assistance reconfiguring the (Mikrotik) router being attached to the ONT 
-- it turned out whoever first set up the Mikrotik (sensibly) assumed 
it'd be seeing IP packets over VLAN 10, but actually the ISP required IP 
packets over PPPoE over VLAN 10 in order for it to work.

Looking around it appears this client's customer's ISP isn't the only 
one that is requiring PPPoE over VLAN tagging over Ethernet for their 
UFB connections.  Is there a reason other than "let's make everything 
look like 1990s dialup so it works with our legacy equipment" for the 
bit/CPE CPU overhead of PPPoE on UFB, including imposing the lowered 
usable MTU and PMTU discovery headaches on the end user?

The only one that really comes to mind is "user/password authentication" 
(rather than needing to collect CPE MAC addresses which seems to happen 
with, eg, Vodafone cable/FibreX).  But it's not clear to me why, eg, 
802.1X isn't used for the user/password authentication in that case; or 
DHCP with some extension to pass a "secret" identifier. 
20-bytes-per-packet-forever seems a large overhead to pay for 
user/password authentication at CPE power on.... (Maybe in the beginning 
there's "lack of CPE support" -- but we're a few years into the UFB 
rollout, and lots of ISPs seem to be supplying their own ISP-badged CPEs 
anyway, which could presumably implement whatever was needed.)

Ewen