By this logic Sony doesn’t get credit for some of their largest IP either.Halo Bungie started development before Xbox bought them.
Sea of Thieves I'll give you this Rare was completely owned by Microsoft for many years before this started development or came out.
Gears of War was developed by Epic who Microsoft doesn't own
Fable This was released before Microsoft purchased Lionhead Studios. It may have been published by Microsoft but it was developed by a non-Xbox Studio at the time
Perfect Dark developed and released by Rare for Nintendo.
Hi-Fi Rush This started development in 2018 before Zenimax was purchased.
Forza I mentioned this but I'll also give you this.
So in like 21 years we have Sea of Thieves and Forza those are the meaningful franchises that they created internally with their own studios in house. Almost all the other successful franchises were developed externally to the company where studios either sold the franchise to the company or were purchased later.
Using your limited criteria Sony only has about 4-5 games they get credit for.
Microsoft has 3-4 with less time in the industry. Once we start adding stuff like Flight simulator and Age of empires they have even more
Seems they aren’t doing too bad