Because if you run, you just run for a long time or keep exercising. In this state, you should not just make your body too tight, because if you wear tight pants, you may feel uncomfortable, and some may cause certain problems to your skin.
Loose pants can protect thighs and help legs. This power may be better. I suggest you wear loose pants, but everyone has different opinions, or to what extent you like them. If you like tights, you can also wear tights or go running.
Because everyone likes different things, you can just test which pants are more comfortable. You can wear this pair of trousers today and that pair of trousers tomorrow, and then feel that you can make the right choice under such circumstances.