There are plenty in the banking/investment industries too to be fair. If you are not as connected to that industry, then you probably just see less of them.
There are probably several factors for the difference though.
1) The lead time in recruitment - having 2-3 years between being recruited and starting a TC means they tend to try and keep themselves a little bit busier. Historically, many either stop or become far less active once they start their TCs. The equivalent grad starting in IB would have a matter of months before starting a job, and are usually in their final year of undergrad/graduate level education.
2) Law is probably a little more of a creative industry than banking and therefore probably attracts people who like being creative in this format. I think law also tends to be a bit more colloborative too, and so maybe that colloboration feeds down into wanting to help people.
3) Banking/Investment is uber competitive compared to law (think 300-600 applications per vacancy while law will typically be a fraction of that) and so maybe it is so competitive, people don't want to help others?
4) The legal sector has far more diversity issues (apart from gender) than banking and so there is more need for "access" opportunities, which I think fuels people's want to make information accessible - in very much the same way TCLA came about.
I think these people have always existed, it is just social media has given them a platform where they are far more visible. Years ago, there would be people doing this on their individual university campuses or via their networks. Social media is a way of monetising the work needed without directly having to ask people for money (which is what individuals used to do in the past - asking for a fee to attend their events for example).
Like anything on the internet, you have to be mindful of the advice given out by people. I regularly disagree with content I see from legal influencers in terms of advice. But then again these things are subjective, despite my experience and training, it is just my opinion on recruitment processes and some people will disagree with the advice I give to people on here. Some of the worst advice I see is from senior lawyers/partners, but I suspect many would consider them to be the most "qualified" to give out advice.