Why do I have to avoid strenuous activity?

Why are you not asking your dentist about this? This is not a website filled with dentists answering every hour of the day.

The Mayo Clinic is famously useful for medical questions. Maybe there are websites that deal with specific dental questions.

But, better, call your dentist's office and ask.
I probably sound snappish, but that is because I can't understand why your dentist hasn't talked with you more in the first place. You have had a major procedure and do rightfully need attention.

Likely the surgeon sees a potential risk of ripping your stitches when partaking in strenuous activity, i.e. work out. Re-stitching can create complications such as infection, excessive scarring, etc.
Listen to your surgeon, take it easy, and rest until you're healed.

Likely.

It is very hard to do heavy lifting without involving the facial muscles. I had the same instruction after surgery for a deviated septum, and that was the explanation. Fortunately, I did have a doctor that explained this.

It may be because your stitches will be affected. It is better to lay off the strenuous activity for a while.