For the company, making use of Open Source code is free labor. That's good for them. You are free to offer that labor or not.
For some developers, it's cool to write code that's used by zillions. That's reward enough.
Other developers release the code for free, but build an eco system around it. They get paid for related work etc.
New developers use it to flex their skills, and demonstrate ability (and then get upset when someone else turns it into something profitable, but that's another story).
Personally I write code, and ship as source, but it's under a commercial license (cause I like to eat.) Other companies have business models around whatever they do.
You are free to act as you wish. Which is great. We live in an economy that allows each his preferred path.
You're right. Many startups open source their products specifically to get free labor, free marketing, or whatever. As payment they release the code they write to you. Whether you think that deal is right for uou or not us up to you.
If you believe you can add value to a company then reach out to them. It's not like they're "making" you work for free.