Interesting question to say the least. I would suggest it's because most Catholics do cherish and value our faith and yearn to do better, but fall short like we all sometimes do.
I've noticed over the years some Singles that don't attend Church as often as they should, but when coupled with someone that does they start attending more frequently. In the same way, maybe being on CM and meeting someone will draw that person closer to the Church and being a better Disciple of Christ.
Not everyone here is an ardent Catholic.
Give them points for honesty. After all, WYSIWYG is better than the alternative.
OTOH, a certain share of profiles on any website are phony -- people who created a profile to check out the site, but have no intention to commit to really using it. By being a little offbeat, they avoid getting contact from serious members.
I can't speak for being a member of Catholicmatch, but I can say that I had left the church for a period of about 10 years. One day I just showed up to student mass at a church on campus. Being around other students who were very devout encouraged me to begin attending church weekly again. I have only been on Catholicmatch for a few weeks, but all of the people I have spoken to only encourage me to become more involved in Church and in my community. It can be discouraging to see "non-practicing" Catholics on this site. However, my hope is that as they talk to more people, they can see that it is possible to grow in their Faith again. We need to help each other to follow God's word.