“why can’t TA’s grade online classes?” They can. But only if there aren’t 10,000 students–unless it’s the sort of class where you can do some kind of automated online assessment.
In the class described by @CardinalFang, there is no possible partial credit, which in my experience is an unusual situation for a higher level STEM class, where the problems are indeed hard. If no one looks at your work, no one knows whether you got 99% of the problem or 0%. The professor (or TA) doesn’t get to know your work, and can’t write a meaningful letter of recommendation for you. I’d be unhappy paying CalTech tuition for that level of personal attention.