I'm not saying it never happens, and I don't want to assume anything about your background, but I think most people who work in software would agree there's no need. Plenty of problems get in on their own.
yep if that were your goal it would be way more cost effective to get a zero day from just not trying that hard with security practices. Not having any security knowledge on the team. Not patching/upgrading dependencies with security bugs.
It doesn't make sense from numbers perspective, there's simply not that much potential for profit there. In general, the sale price of a zero-day or ten in some popular product is tiny compared to, for example, the marketing budget of that product.
That money is significant from the perspective of a particular employee (i.e. if they personally would get the money) or for a specialized consulting company, but it's a drop in the ocean for the large companies actually making the products. So we should expect some backdoors intentionally placed by rogue employees (either for financial motivation or at the behest of some government) but not knowingly placed by the organizations - unless in cooperation with their host government, not for financial reasons.
