This is a good point. I think it's just due to convention --- people tend to put terms involving x only at the front, and in the textbook by Cox et. al. that I referenced they use this convention as well.
If you can't algebraically solve the univariate polynomial, you can still get some solutions to the system as long as you can obtain some solutions to the univariate polynomial numerically.
Author here. Gröbner bases are powerful tools for us to understand polynomials. I started on this article when I encountered them during research, and decided to write down some notes. After a few months, these notes turned into this blog post. It's quite a long read, and let me know if you have any questions.