Most vtuber managers do nothing but things the talents can already do themselves, they simply take the pressure of those decisions off of the talent's shoulders so they can focus on streaming. This is a very low-skill job, and compounds with the issue that there is no standard of a 'good manager' versus a 'bad manager'. How do you tell the difference, exactly? Unless they're blatantly incompetent, it's extremely hard. If you grow, was it because of your manager, or was it just your natural talent getting a bigger audience? If you fail, did your manager screw up or were you just unlucky or genuinely untalented enough that you didn't attract an audience? What metrics do you use to calculate this?
What even is a manager, exactly? Ten different people will all have different definitions of a manager. There is no universal standard, and therefore it is not a good career choice. Very few jobs will be gatekept by actual business qualifications, and those that do won't be paying anything more than you could get by handling 5-10 clueless indies with severe social anxiety who are happy to give you a 10% revenue cut to write them the odd reply to a sponsorship email.
When you look at the industry from an analytical perspective, you quickly realize there are no conditions that foster a pool of good managerial talent. The money isn't there. The support systems aren't there. The qualifications aren't there. Every job will have different requirements. Most of the big opportunities will be bi or multilingual. I would go so far as to say there is not one single vtuber manager out there who couldn't make more money using those same skills in another industry.