Humans, even talented mathematicians, have limited context. A big part of any mathematics text is abstraction for the sake of understanding. It can be confusing at first, but once you learn how to read mathematics with these abstractions in place, reading everything spelled out with great verbocity and pedantic accuracy is frustrating and tiring. You're forcing your eyes to parse and interpret a whole bunch of symbols that your brain doesn't need.

Of course, some mathematicians take it too far and use these abstractions to obfuscate and prove how smart they are. Like everything, it's a balance.

I personally wasn't a fan of this particular shorthand when I read this book but I got used to it quickly.