It's actually quite strange after all the time even the wealthy merchants haven't found out the secret about magic. In RL there were always impoverished nobles around, ready to sell all kinds of secrets or rights to those with enough money. After all, although being born in a noble family did grant things that couldn't be bought with money, it doesn't change the fact money is still needed to live and especially to live lavishly. However, simply being born a noble didn't automatically mean a lot of income. There were all kinds of nobles and all kinds of circumstances. Many nobles couldn't handle their finances or war or other catastrophes robbed them of their land and wealth. Thus wealthy "commoner" merchants, who could handle their finances splendidly, were able to buy their way into noble lineages by making a deal with one of the fallen noble houses. However, it looks like in the world of this work the nobles have been able to keep their secrets with 100% efficiency.
To be honest I feel like Main has suffered long enough. This is already chapter 30 and she hasn't got a single clue or hint, let alone started working toward gaining control over her mana. Instead she's giving up. If this series wasn't otherwise so good, I feel like I'd have started hating this a while ago already. In fact I'm kind of displeased by how she's unable to apply previous life knowledge in some specific ways yet is superbly able to apply it in others. She's like one of those boring people with no imagination whatsoever but who can do just fine anything that doesn't require any imagination. That is, people who couldn't invent anything new to save their life, yet can recreate old inventions just fine. Totally unable to think outside the box.