> So your 100 meg line has a "cost" of $100 for shit-tier transit

Wholesale rates from quality providers are about $0.30 per Mbps per month at 95th percentile. For paid transit, that is, and Comcast does a lot of peering which brings the cost down for them.

So that's $30 per month for your 100M line, even if you run it full tilt all month and manage to hit only paid transit routes all the time. That's not too bad and most people use only a small fraction of their theoretical monthly throughput.

Honest question: what's stopping every larger apartment complex from getting a 10gbps line and paying for bandwidth, then offering that as an Internet option to residents (delivery via cat5)?

Two things:

first, the costs cited above are for handoffs at major peering points, private or public. Pulling 10Gb/s across to your apartment complex is going to add a lot to the cost, if it can even be done (right of way issues).

second, the expertise needed to realize that you can do this and then provide support for it is neither common nor cheap.

If you had a tower on a nice highly-visible location (say, the giant Jesus statue in Rio) and could do line-of-sight laser links, that would be an extremely cost-effective solution. Sadly, that sort of opportunity is rare.

Not everybody wants to be an ISP. Not enough profit in it to make it worth it for the manager. Lack of backhaul and/or cost/difficulty of connecting to backhaul. Existing long term contracts of tennants. High turnover of tennants. Lack of scale.

some do, i used to live at a place served by https://webpass.net which wires up larger apartment complexes with ethernet and runs as many fiber drops as necessary. They only operate in a limited area with specific kinds of buildings though.