If only it was really as easy as you paint it... PS360 generation was very different then today, games didn't cost a fortune to make and didn't need 4-6 years to develop, 3-5M units sold was a huge success back then, hence the publishers weren't afraid to experiment, and as a result the generation spawned a ton of new, unique IPs people still talk about and want sequels/remasters/remakes. But with PS4/XB1 the dev time and cost skyrocket, so much that it's really hard to get back that investment, let alone make a profit. That's why that generation was so safe, so boring, almost everything was the same old IPs or copied formulas with added open-world design, no innovation whatsoever, and your AC example is the very best proof.
Which speaking of, AC with dozen installments already showed it has indeed stood the test of time, GoT has yet to prove it. But that's just one game, what about The Order 1886? Driveclub? Knack? Dreams? The Last Guardian? Days Gone? Death Stranding? Examples are there, it's NOT easy to establish a completely new IP in today's landscape.
Now, while your math is correct and instead of buying other publishers Sony could fund tens of new games instead, there's a huge problem with that, something that cannot be bought - time. With a limited number of studios they can create only so many games within one generation, so to buying an already established studio is the fastest and easiest way to expand capabilities, while starting a new one from scratch is the exact opposite - it takes a lot of time and money just to build a studio, then it has to make a game, and at the end of the day you're not guaranteed it'll be all worth it.
So you just buy a publisher, its studios, its games, and what's more important - you get all its community within the purchase. If for example Sony bought Ubisoft - that's a lot of great, popular IPs, Tom Clancy's license is there as well, but there's also gigantic community who who plays Ubisoft's games on a daily basis - Assassin Creed, Ghost Recon, R6, For Honor, The Crew, they also published Brawhalla, so with such an acquisition you could potentially lure some of that crowd inside your ecosystem where they'd be leaving you their money, which is the ultimate goal of every platform holder.