I think this is partly because big OEMs doubt (used to doubt?) AMD’s ability to consistently deliver product in the kind of volume they need. Partly it’s because of Intel’s historically anticompetitive business practices.
> Multiple reports, citing sources at laptop OEMs, have covered what is said to be poor support, chip supply, and communication from AMD with its laptop partners, leading to generally poor execution. Chip consultancy firm AC Analysis says AMD's shift of focus to AI and data centers has led to a "'Cold War ice age' in relationships with OEMs," leading to a loss of trust from its partners.
https://www.tomshardware.com/tech-industry/amds-laptop-oems-...
That's because Intel bribed OEMs to use only Intel chips