Isn't it cheaper to do the last mile wireless...
Yes, if you're the only one using it. Otherwise it's shared bandwidth with everyone else, whereas wired bandwidth is, to a first approximation, all yours (assuming your ISP has properly sized the pipes).
If you've got 100Mbit/s then you lose some of that because radio is half-duplex compared to the full-duplex of a wired system that can (in theory) stream 100MBit to you while taking the return traffic. Radio has to stop sending so you can send the acknowledgements. Then there's the overhead needed for each radio burst so the RX is in sync with the TX before the real data starts. Then there's all the neighbours also wanting some of that airtime.
Point to point wireless is possible, but costs a lot more and may exceed the cost of installing a wired channel.