That’s an erroneous impression/assumption some people have. Because of the extraordinary rigor in its undergraduate program (which, BTW, was even more so in earlier times), some Caltech alumni struggled and didn’t do well when they were there as undergrads. Caltech undergrads actually are treated better and enjoy more support than its graduate students, and arguably than students anywhere.
No TA or grad student teaches any course at Caltech. TAs are only in charge of recitation sessions. Caltech professors are among the easiest to get in touch with for research universities, as most classes are very small. Also, Caltech doesn’t offer an aerospace major because it believes that discipline is too specialized to be offered at the undergraduate level that can meet what is expected of a Caltech graduate. It thinks the student would be better off building a more solid foundation in underlying sciences and engineering, and then specialize later in a graduate program. When it has to choose, Caltech does have a bias toward foundational understandings over practical applications.