Also, bigger ships are bigger targets. Band of happy pirates in cutter or small sloop, attacking smaller coastal traders, smuggling between islands and similar shenanigans may fly under the radar. Even if you come across ship of the line, you can usually outmaneuver it and hide somewhere out of reach. But with corvette, frigate and similar medium to large sized ships, you can't really do that. Plus, ships are expensive to buy and to maintain. Larger the ship, more expensive it is ( and it groves closer to quadratic/exponential). Plus, people get attached to their ships.
Pricing wise, used a bit of chatgpt magic to create 3 ships, sloop corvette frigate. Fully equipped they are priced at 22k gp, 61k gp and 170k gp. Even if players get that kind of money, you can't really buy it that easy.