I agree it's a mistake - once in a while, the annual is even integral to the online plot.
Annuals actually have had variants the past two years.
It is true that during the Rebirth era, annuals virtually never came with variants.
DC has leaned very, very, very heavily into variants and ratio variants during the last year or two - more and more every month.
I was surprised when annuals also started coming with variants during 2021. That year, there was no annual that did NOT get a variant. This was the 2021 list that all came with variants:
Action 2021 Annual #1
Batman 2021 Annual #1
Catwoman 2021 Annual #1
Detective 2021 Annual #1
Flash 2021 Annual #1
Batman/Superman 2021 Annual #1
Green Lantern 2021 Annual #1
Harley Quinn 2021 Annual #1
Joker 2021 Annual #1
Justice League Dark 2021 Annual #1
Midnighter 2021 Annual #1
Nightwing 2021 Annual #1
Robin 2021 Annual #1
Suicide Squad 2021 Annual #1
Superman Son of Kal-El 2021 Annual #1
Wonder Woman 2021 Annual #1
There have been fewer annuals in 2022, but most still with variants. Here are the ones with variants:
Action 2022 Annual #1
Batman 2022 Annual #1
Harley Quinn 2022 Annual #1
Justice League 2022 Annual #1
Nightwing 2022 Annual #1
Wonder Girl 2022 Annual #1
There were just 3 annuals in 2022 that did not get variants:
Detective 2022 Annual #1
Flash 2022 Annual #1
Batgirls 2022 Annual #1