Politics.  City, county and state elected officials in bed with, related to, or getting campaign funds from illegal landlords.  This is an easy fix.  Enforce the laws related to vacation rentals vigorously.  Pass ordinances establishing that online rental advertisements (especially air bnb) establish probable cause for searches, and go out and check.  UNANNOUNCED.  Make the fines heavy so that it is not economically worthwhile to simply just rent, violate the law a, pay the fine and collect transient rent.  Do the reearch necessary once illegal rentals are identified to find out whether GET, property taxes, income taxes, and transient accomodation taxes are being paid.  This situation does nothing more on balance than harm law abiding residents (who, e.g. abide by zoning laws, don't skirt GET, income, property and transient accomodation taxes  are being paid.  The City really wants to cure the housing problem?  Do all that instead of spending millions walking around talking to homeless