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.
