This is a question that will be relatable to many patients and physiotherapists alike.
The table below created by Tom Goom (2018), provides an excellent insight and illustration as to why certain activities have to be avoided or reduced during recovery from knee related pain/injury. It shows that the load placed through the patellofemoral joint- which is the joint between the kneecap and thigh bone- is significantly more during activities such as running or squatting. Therefore, this explains why physiotherapists may advise against returning to particular activities or sports until a patient has reached a suitable stage in their recovery.
Reference: GOOM, T., 2020. Knee [online]. RunningPhysio. [viewed 01 March 2020]. Available from: https://www.running-physio.com/category/running-injuries/knee-running-injuries/