For many reasons.
I find some creators put a lot of ego into their products, not that they are egoists so much, but doing something on your own can be emotionally challenging. I don't mean 'ego' in a negative way - just that when you are on your own you have to make so, so many decisions. APIs, compatibility, etc. etc. you're often a little overwhelmed, you often have to 'just do it' and 'go with your gut' in many areas. Which may or may not be a good practice/attitude for a Google employee.
They might have a stronger sense of the 'outcome' (i.e. what it does) than the capacity for the underlying code. Maybe it 'does the job' but is poorly written? Which again, might not work in Google environment.
... and working on a very large team is night-and-day from working on your own.
I think we all want to 'feel it is unfair' that such an important contributor somehow 'can't get a job at Google' ... maybe he's worthy, maybe not, but I can totally grasp someone mighn't be a good contributor at Google.
Maybe it's easier to see it from a non-technical standpoint: it's often very difficult for founders of their own companies to get along in a place like Google. I think that's easier for us grasp. The same thing can apply in the technical domain.
But when it comes to an interview, I cannot get a job several levels below my own (without referrals).
With referrals, I am offered the moon and the stars, complete pick out of a tremendous number of amazing jobs.
But any random interview, and I totally bomb it. I wonder how many out there are like that as well. It feels rather strange from my perspective, but maybe it's fairly normal?
Look at how many failed or poorly designed products and libs Google has released.
Maybe this is what happens when an industry is built around worshipping whiz kids. If CS in SV was about venerating NASA/aerospace code, less flashy but very very stable, well-engineered stuff that lives are dependent upon, it'd look a lot more like EE and other more traditional engineering fields.