Can you count? Do you know all the math signs and how to do PEMDAS? Then you know all the things you wouldn't otherwise know without a guide or a calculator.
Need to instruct a student who asks you a stumper? You just need to know how to formulate and can use a guide or a calculator to help you if you had to.
Not good at math without one? You could chalk it up to everyone having days like that, as it's not like you could expect a school to remove you because of brainfart.
If you became any other kind of teacher (English, history, science, etc.) you would need to build up a starter pack of knowledge on the subject, such as different obscure forms of grammar or what happened during the war of 1812. With math, you aren't expected to be perfect since it's a circumstantial mental skill, and when you subtract that from the equation (no pun intended), you can have any skill level that allows you to pass the instructions on how to perform equations to the students.
You also don't have to know what everything means, since there are things that we take for granted only advanced math classes will teach you.
You seem to be under the mistaken impression that all that is needed to teach something is to know it, thus ignoring what teachers are taught to do when they get their teaching qualifications.
It would need to be a calculator which gives correct answers, which therefore leaves out some Texas Instruments calcs and almost all e-calcs (welcome to what happens when programmers don't check their Maths first when writing a calculator). The Windows calc in Standard mode says 2+3x4=20. Turns out teachers need to know how to do Maths without a calculator.
And learn how to teach, just like Maths teachers also have to do (well, not in the U.S., but that's another story).
No, you have to have a skill level of being able to teach it to all types of students. If you have to teach students that 1+1=2, and you don't know why 1+1=2, and a student asks you why 1+1=2, what are you going to do then? If teaching was nothing more than passing on facts then we could just give every kid books on the subject and do away with school.
You got that the wrong way around. What's taken for granted by advanced Maths classes is what you are taught only in lower Maths classes. Welcome to a whole bunch of Professors don't know the correct answer to order of operations problems because they don't teach it and have forgotten the rules ๐