The 95% who don’t trust the 5% who do. If there is a backdoor in open source projects it gets known very quickly.
Apart from that, open source projects usually are not for profit, they have no reason to add random unneeded data collection features for example.
Could they please explain why a laptop should not be able to scale content on third party monitors without lowering resolution? Why it shouldn’t be able to connect to more than one monitor? Why we can’t have a toggle for (insert random unneeded feature here, like only minimizing programs when clicking on the red x button that should close them). Why their tablets and phones aren’t able to send things via Bluetooth? Etc.
Try GrapheneOS (on a Google pixel, ironically) if you truly want privacy