I’m curious, how are those scammy? I always rent the car in advance and never had any issue, I’ve done that in Spain several times but only in the islands (Canary and Balearic).
We booked one in France through Avis. Rocked up and they wanted either a €2000 refundable deposit or a €350 non-refundable waiver. We were deep into a home renovation, couldn't afford the refundable option if we also wanted food on holiday, had to take the hit.
That's over the top of a €950 10 day rental for a 3 year old Seat, its gearbox nearing the end of its short cruel life.
And we were treated like this because we'd used a reseller. The person before us didn't have our experience and the guy there was happy enough to explain how they hide details like this from comparison websites and everyone assumes they'll get the standard rental agreement. Loads of people get stung on the waiver.
I'm guessing the 'waiver' is for their enhanced insurance, which means you don't pay anything extra if there are any damages.
I've rented hundreds of cars in Europe over the past two decades, and never had an issue. I booked through whatever site is the cheapest (which often is their own site, once you sign up for the loyalty programme). Never took the extras they try to sell. Car seats are carried by airlines for free (even Ryanair), so you don't need to pay for those.
What made it feel like thievery was we knew nothing about it until we were in France with two kids who had just had enough of travel, and we were already an eye-watering sum on that we wouldn't get back if we tried to shop elsewhere.
Again, the direct customers didn't have this. They had been asked these questions, made these choices as part of the initial order, where they had time to prepare financially, or shop around.
It's in this way very similar to how some airlines feel they can double your flight cost as you are literally stepping onto the plane. You have no choice.
Especially for car rentals where even a single paperwork error can cause a huge problem if an accident occurs.