their, not they are - sorry, I can't get past that.
No, the regular number fonts aren't illegible. I'll paint with a broad brush - many color rush jerseys have colored numbers on colored jerseys. That combination is horrible for viewing by broadcasters and fans. By using a standard block font rather than anything else, you make the numbers as legible as possible.
For normal jerseys, the contrast between number and jersey is greater, which allows for custom number fonts to be used and still be legible, relatively speaking. Certain numbers on certain fonts (hello, Philadelphia) are just flat out tough.