It's logistically very difficult. All ProTour riders will have at least one spare bike. The teams will also have at least 20 spare wheelsets available. For a typical one-day like the Paris-Roubaix, that means at least 400 bikes and a thousand spare wheels. These bikes and wheels all need to be checked and maintained before every day of racing.
Cycle races often start and finish in small towns, so the teams may have to stay in hotels some distance from the course. It would be impractical to find a suitable building for parc ferme at most races and it would be costly to erect temporary facilities.
The professional racing calendar is extremely busy, so a team mechanic may be travelling continuously from late January to early December. There simply isn't the time available for a parc ferme.
I think the current strategy is perfectly satisfactory. Riders are immediately intercepted after the finish line. The race leaders and some randomly selected riders will have their equipment inspected, in the same way that they are tested for doping.