As someone who works as a government underwriter, I can tell you why.
Lenders are getting besieged with refinance applications, and that, combined with the fact that appraisers are now very hesitant to go and do appraisals, which require outdoor and indoor inspections, are going to result in delayed loans, which costs the lenders money to extend rate locks. They are basically trying to stem the tide. The company I work for had to do the same thing.
And since refinancing is strictly a rate driven market vis a vis a purchase market which is more predicated upon relationships and delivering timely service, these lenders don't want to be sitting on all of these rate locks that will cost a significant amount of money to extend, if there is no indication of when these loans can get done.
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We are getting delayed appraisals right now on a majority of our purchase loans, but there isn't really anything we can do about it. We can't force appraisers to go out there.