Probably because they don't care with stay current: the PS4 isn't going to change much after initial development, so why should the base OS? Similarly now that the PS5 has shipped, I doubt there will be much OS development there.
If FreeBSD (or Linux) weren't around, then they'd simply purchase/license an embedded OS (QNX?) and use that.
If FreeBSD (or Linux) weren't around, then they'd simply purchase/license an embedded OS (QNX?) and use that.