He doesn't have a steady stream of income. Not that $2.5B is anything to scoff at, but if he wants something, he's gonna gun for it, now, because his money can "run out", so he's gonna get what he can, while he can. Jay-Z and Beyoncé will be raking in a half billion, each, a year off CDs, tours, merch, and NBA stuff (in Jay-Z's case) etc. for years to come. They could have easily bought this house if they wanted but they just gave up. And they could easily buy a more expensive house and their bank account will be, essentially, unaffected.
Notch's bank account dropped $70M and it's not going back up. Just saying all this for people thinking that Notch's "Minecraft money" has made him "more powerful" than Jay-Zand Beyoncé because he was able to outbid them on something. It'd be different if he was getting residual income and royalties off the IP, but that's all Microsoft's now.