"That beam is WAY more powerful than the sun per unit area. If not, we'd just use the sun."A common misconception, but untrue.
What matters isn't power per area but cost per kilowatt-hour. To collect solar power on Earth you need a lot of expensive components (photovoltaic cells), plus you probably need to track the sun, and even so you'll have huge losses due to night-time, weather, winter sun angle, and so on. In comparison, a rectenna array is just a bunch of little bent pieces of wire. It's almost trivially inexpensive to build, and in some cases it can live side by side with other land uses. Even if the power density is only a few hundred watts m^2 (compared to over a kilowatt/m^2 from the sun at peak power) that power can be converted with upwards of 80% efficiency (compared to much less than 50% efficiency) and the full amount of that power is available 24/7 through every season. The hypothetical 1kw/m^2 at noon on a summer's day actually rounds out to only a tiny fraction of that over the entire year. Meanwhile, a rectenna converting 100 W/m^2 of RF power can end up producing the same average power as a solar installation, except that power is always available and the cost of the ground component is a teeny, tiny fraction of the ground solar installation.
That still leaves the cost of the space component, of course, which is decidedly non-trivial. But a big advantage is that the space component can have a very, very long service life and can also serve various markets (especially remote regions where power delivery is difficult) which can provide the up-front high-profit business to pay off its cost.