if you asked AMD execs they'd probably say they never had the money to build out a software team like NVIDIA's. that might only be part of the answer. the rest would be things like lack of vision, "can't turn a tanker on a dime", etc.
according to public information NVIDIA started working on CUDA in 2004, that was before AMD made the ATI acquisition.
my suspicion is that back then ATI and NVIDIA had very different orientations. neither AMD nor ATI were ever really that serious about software. so in that sense i guess it was a match made in heaven.
so you have a cultural problem, which is bad enough, then you add in the lean years AMD spent in survival mode. forget growing software team, they had to cling on to fewer people just to get through.
now they're playing catch-up in a cutthroat market that's moving at light speed compared to 20 years ago.
we're talking about a major fumble here so it's easy to lose context and misunderstand things were a little more complex than they appeared.