Why do doctors recommend not exercising for a month after cataract surgery? is it old school advice?

Somewhat: Assuming an uncomplicated cataract surgery with modern techniques in a patient who is otherwise healthy enough to exercise, I do not place a restriction past the first 1-2 days. Contact sports should be avoided for the first 1-2 weeks. Swimming should be avoided for the first 2 weeks.

Old school: I recommend light activity and lifting up to 8-10 pounds for 1 week and no heavy lifting (more than 40 pounds) for 2 weeks. A month is not necessary with small incision uncomplicated cataract surgery.

SAFETY: Most cataract wounds are self sealing and do not require a suture. Placing undue stress to this wound before healing may cause a wound leak. Every cataract surgeon has had to take a patient back to the or at some time to fix, or try to fix an eye in a patient who did not heed the restrictions. Don't risk a lifetime of good vision by doing too much in the post operative period.