I explain it this way:
When a lender escrows for a borrower, the lender is taking on a fiduciary responsibility. The lender takes on risk of calculating proper amounts, and making timely payments. Borrowers can have voluntary escrow accounts and stop this process at any time, but if the lender is taking on these responsibilities, they aren't paying games. Thus, borrowers can't start/stop, make escrow payments some months but not others, etc. You're either in or you're out.