Required by who?
They're definitely not a requirement. People make it to the airlines without them, and even without a CFI at all, frequently.
But, they do open yourself up to more opportunities. More flight time that is more valuable (multi time, probably more high performance/complex time too). They'll make you more marketable for flight instructor jobs, possibly allowing you to command a higher salary as well. And, for when the time comes that you get to an airline interview, you'll have more on your resume and more stories to tell the interviewers.
So, long story short, no, you don't have to have them... but I recommend them strongly.