On that note how many random programmers out there at any one time look at these open source codes so the random programmer can do a charitable service by investing his/her time for free by looking at these open source codes to find bugs/errors to notify the developers to patch up? Do such free beta testers exist in the crypto world?
There are many incentives for helping open source projects; you're right that almost nobody does it 'for free' 
Some open source projects (like hardware wallet manufacturers who put their firmware code on GitHub) offer bounties for finding security bugs. So in that case, purely financial incentive.
IT security researchers analyse the Bitcoin codebase, or e.g. Linux kernel to write papers about the vulnerabilities they find. This helps their CV and recognition in the industry.
These are just two possible incentives and of course there are also the selfless helpers who maybe retired early due to crypto gains or just have lots of spare time and help the community by looking for bugs, similarly to the Bitcoin Core devs who also don't get paid for their time working on this project.