Because these servers are Simulation servers, use by the members of the community in & out. To ensure safe atmosphere and to keep it as close to real life as possible, these servers have restricted speed limits.
We do have Arcade servers exactly for these reasons, if you wish to not be restricted by speed limits and want to have fun with your friends, There are 2 Arcade servers available with No Speed Limit restrictions and No Collision with No Road Rules enforced.
Regarding TruckersMP 10th Anniversary, we have just concluded these events which you may have missed.
We had a No Road Rules server opened for this event to allow players to enjoy crashing into each other while being able to access police cars as part of the festivities.
There is a archived post with more information on the event and what was conducted.
There is even a fantastic video showcasing the event, on a special made island by our in-house add-on team.