being a member of the CPC comes with duties and other obligations because you represent the state. For example you should be married before joining or at least in a relationship because they don't want you to date around due to the power implications (you being a member and the people you date not being members).

It's not like joining a party in liberal democracies where you just pay the dues and then do whatever.

I don't fully know what it entails but the CPC has 98 million party members. For a population of 1.4 billion that seems low, but you have to remember that not every single person has the ambition to be politically involved. Many people just live their lives without being member of a political party. And even then, almost 1 in 14 people being member of the biggest party in the country is pretty massive.