Biochar is a totally valid way to fix carbon, yeah. There's not enough trees in cities to make a dent, but if you think bigger, there's potential.
However, nobody wants to personally pay for that, so there's no way to employ people doing it. It's kind of the same answer to every climate change inaction question.
It's not even a money problem. In whatever non-market economy, there still wouldn't be the will to divert resources to it, because 90% of people just don't care that much.