Between that too-easy-to-pass-up profit optimization, and the marketing benefits of deceptively low list price, it's become pretty pervasive. And isn't limited to actual resorts – I've had bog-standard business hotels in the suburbs tack on a $30/night non-waivable "destination fee". Luckily it got to a ridiculous level recently, triggering a lot of media backlash and class action lawsuits last year[1][2].
It's also losing effectiveness, as things like online booking websites start displaying all-inclusive pricing and renegotiate their commission structures to be inclusive of mandatory, non-governmental fees. Between that, all the negative sentiment, the spate of lawsuits and legislation its triggered, the fees are hopefully on their way out.
[1] http://killresortfees.com/
[2] https://www.cnbc.com/2020/02/07/resort-fees-being-addressed-...
Also, in my encounters, resort fees are in lieu of being a boarder. The resorts by me, Miami Beach, charge visitors for access to the pools and spas, not their guests.