Because life for 14-18yo’s is generally predictable.
Life for 55-80 yo’s is also fairly predictable–retired or quasi-retired, aches and pains, lots of time and not lots of money.
The difference between an 18yo and a 25yo can be huge. An 18yo is likely living at home and going to college. If they are employed it’s typically “under” employed. Typically the 18-22 crowd can date but it’s less than desirable and often fraught with issues.
But a 25yo? They are more likely to have a good job. They may have plenty of extra spending money. They are in a STABLE place to date. They are looking at buying a home, settling down and getting established in a career.
Closer to 30 you’re dealing with people with toddlers. As a young adult I was happy to deal with parents with toddlers and their limitations. However many 18-22yo’s were not. They wanted to work in soup kitchens and hike mountains, hike-in camp backwoods overnight, etc…not exactly family friendly. They tend to be available after 7pm–but that’s bedtime to those with kids.
It can be very hard to enforce the young adult Catholic part and not simply a singles/dating group. Too often great groups are fractured by love triangles, quadrangles and all sorts of dramatic nonsense. It is much easier to enforce no dating/discreet dating in teen groups.
So the answer?
Honestly, diocesan level events, greater support for 18-22yo college kids. Events aimed more at 23-40yo’s that are a mix of single/dating/young family events.
Young families often get the shaft, too, so this isn’t just a “singles” problem.