Storage prices. I mean they already get it at a discount but it's the easiest way to "cut costs for the consumer" without heavily affecting performance. And Sony has been really good about giving you options to personally upgrade it. Do I wish I could just pay more for a bigger size version? sure. But this is one way they get their hardware to the "console price"
And not for nothing. Low storage has been an issue on consoles since like always. Looking at you 360 hard drives.
Its not technical limitations because they sell larger ssd's that are perfectly compatible with the ps5.