Because you won't get money from OSS contribution for most part. If your priority is money, then OSS is not the best option unless you have a clear strategy of monetizing it.
Big companies have an Open Source Program Office (OSPO) where community / OSS skills and licensing knowledge mean you can help the company both consume OSS and contribute back to it.
But still, it's something else to say
"OSS doesn't make money" (which is controversial, if not wrong)
instead of:
"OSS doesn't make money if you don't have enough time to build the side services and market it because of lack of time meanwhile maintain[ing] a job"
(which most can agree with)