tl;dr: It doesn’t really matter how a foundation incorporates; what matters is the actual governance.
A typical response to the creation of new open source foundations is to decry them as “vanity foundations”. In a few instances, that may be true, but I think as a generalization it is not correct.
Usually, companies think first before spending significant money on something, in particular if it is of high visibility and might turn into an embarrassement. This doesn’t mean they always fully understand what they are doing. In fact, I believe that the understanding of companies of what open source means to them and how they want to support and steer its development is ever evolving. After a learning period these “vanity foundations” might just end up with a project and governance structure like the ASF’s.
If you are asking why these foundations didn’t start out as Apache projects then, the answer is simple: When they started out, they had not seen their future yet. Corporate thinking steered them into a direction that made the ASF in its current form unappealing. Rather, they decided to create their own (typically) 501(c)6 “for-member-benefit” organization, in contrast to Apache’s 501(c)3 “for-public-benefit” incorporation.
While it might be better to start as a for-public-benefit non-profit right away, eventually, it doesn’t matter. While there are examples where corporate “shenanigans” hobble proper open source, there are also examples where, in the end, it didn’t matter. The Eclipse Foundation is an example of a for-member-benefit non-profit that is guiding the development of open source software that is not fundamentally different from the ASF’s.