> I don't understand why there isn't a dedicated line
Obviously, because it would take away capacity for the regular users?
It's among the most heavily used lines in the world and tourists are probably a single digit percent of users, or less, so whatever funds available are better used to increase global capacity than to provide a better experience for a few people taking the plane.
CDG is also one of the busiest airports in the world and is used by French residents, not only tourists. If they can fill trains with airport traffic then obviously the demand is there, and that makes the other trains nicer for commuters who no longer have to step over luggage and get bonked by giant backpacks.
CDG is comparable to Heathrow. Before the pandemic, Heathrow Express carried 17k passengers/day. That's a bit over 100 passengers per departure, or about 1/6 of the capacity. It turns out that even the busiest airports in the world are barely busy enough to justify dedicated rail service.
Dedicated airport trains are expensive, because the fixed costs are high and somebody has to pay the bills. Their passenger numbers are often naturally low, because they only make a few stops, which are not where most people are going. Metro lines and local / regional / long-distance railway lines that would exist without the airport are always more cost-effective. For many people, they are also more convenient, because they go to the right direction and stop at the right place.
Obviously, because it would take away capacity for the regular users?
It's among the most heavily used lines in the world and tourists are probably a single digit percent of users, or less, so whatever funds available are better used to increase global capacity than to provide a better experience for a few people taking the plane.