To be fair, yes, the proprietary nature of a particular chip is bad for open source, but on the other hand, the marginal cost of a chip is not zero, like it is for software. There's the chip fabrication costs which only goes down in quantity. That means there's an inherent risk in developing a new chip.

However, if you're that concerned about it, there are open source designs which create free truly random bits, so you are free to choose a design that meets your need.