A 200 day lock is a call option on a loan, the price of the option changes all the time. What you're saying is that local banks/CUs would offer such an option for free. Someone has to pay for the option - in the scenario you describe, the bank/CU would pay for it by losing liquidity of its assets, then having to mark them down.

Rate locks are backed by rate swaps. The cost of purchasing a rate swap ultimately comes out of your pocket in the form of additional rate on the loan (the lender can add overhead of course). The cost of rate swaps has doubled in the past 3 months and quadrupled in the past 18 months. I believe it's currently around 3% on 10 year loans, so a 60 day lock on a $500K 10Y mortgage would cost about $2500 while a 200 day lock would cost over $8000.