The pipes used are also mass produced and manufacturing circular ones is also usually easier and cheaper than cross-sections with other shapes. Obviously there are other important considerations like the ease of tunnelling and the strength of the pipes when buried underground as well.

However, one thing that none of this depends on is a circular cover being the only shape that can't fall down the corresponding circular hole. That was the standard answer that way too many companies that used this particular interview question wanted and it was wrong.