> they could get the same data with more reliability by just running their own ADS-B receiver, without having a dependency on a third-party.

Setting up an ADS-B receiver is indeed very cheap. Less than 100$. That's what many people, both aviation enthusiasts and ham radio operators, do for fun.

The problem is, do that on an airport? You'll now need permits to install the antenna (needs to be covered in the lightning protection system and even if it's just a passive receiver probably someone needs to sign off on an antenna being added). Fire code means you'll need approval and specialized people to run the cable (you need to drill holes in fire walls). Maybe there's some law or regulation requiring approval or causing a paper trail (e.g. in Germany, all electrical appliances have to be isolation-tested and visually inspected every two years by an electrician). Doing that the proper way is an awful lot of work. And by that point, someone will notice "hey, a Raspberry Pi? An RTL-SDR stick from eBay? No way that is certified to be used in a safety critical environment", killing off the project or requiring a certified device costing orders of magnitude more money.

In contrast, a privately owned laptop, tablet or phone with the Flightaware app? No one will give a shit about it unless someone relies on FA too much, causes an incident and that is found out.