Frankly, the Church doesn’t provide a whole lot of support to young adults. If you’re a kid, a teen, a college student, married, or a parent, there are lots of things going on to help you / support you / etc. in the Church. There are CCD, RCIA, Newman communities, marriage encounter, etc., etc., etc., etc. But 20- and 30-somethings find very little going on at Church for them. There simply aren’t any programs aimed at meeting their needs. So they go elsewhere.
There are lots of groups that have no specific age focus (lots of the service groups, for example). But, when you join them, they’re frequently made up of people who have very little in common with young adults (Cursillo may be a great movement, but if your weekly meetings consist of a dozen 50-somethings talking about how God is present in their lives, eventually it starts to feel irrelevant to you: you don’t have a spouse, or kids, or retirement concerns, and no one else remembers the stress of dating, finding your first job, settling on your career, etc.). At least in my area, until very recently you really had to work hard to find Church activities with people in the same age range. Even now, the local parishes have very little aimed at young adults; you’re mostly stuck with diocesan activities. If you can travel there.
There is the Mass, of course; there will always be the Mass. But, if you’re spending your entire social life with others of your age, you start drifting away. The weekend trip doesn’t get back in time for Sunday night Mass. The dance lasts late, and you miss devotions. The group going to dinner conflicts with Eucharistic Adoration. You decide to go to work on Sunday instead of church, and no one calls you on it; your friends are all non-Catholics, and the priest doesn’t know your name. If you aren’t careful to make a lot of Catholic friends, it’s very, very easy to drift away. Not that you ever stop being Catholic; you just don’t show up.
Then, of course, once you’re engaged, the Church suddenly seems to care about you again. Because we have programs for that.
Unless, in the meantime, you started attending that nice Baptist church down the road, whose members make you feel so incredibly welcome when you show up…