Please explain why full liquid phase lasts so long

There really isn't a reason other than your surgeon's experience with what works post-op to yield success. My post-op plan never had a liquid stage. I moved directly onto purees day 1 post-op. Every plan is different.

You might put a call into your surgeon's office and ask them for the rationale behind their plan.

Every surgeon's office has different recommendations/requirements for both preop and post op plans.  There is no "one plan fits all" program.

Do everything in your power to strictly follow your post op plan.  We all became fat thinking "one little piece of cake won't hurt" and look where it has us.  You've undergone major surgery to give you a reset on your weight.   I know it's boring drinking fluids at this stage, but the time will pass and you will be grateful that you were adherent.  Now is the time to set yourself up for long term success.

Hang in there.  It gets better