Because of the amount of bending your new knee will have to do, at the same time as maintaining balance. Initially, your knee won't bend well. It is possible, though.
If you read the article Pumpkln left you, -
Stories of amazing knee recoveries - you will see that it is indeed possible to resume a normal life, once your knee has healed properly, which can take as long as a full year.
When can I start taking walks, ride a stationary bike, lift arm weights?
You'll start walking right from Day One post-op (with assistance), and you'll gradually increase the amount of walking you do. When I started walking outdoors, I used to time my walk, starting with 5 minutes out and 5 minutes back, gradually increasing the time and taking note of how my new knee reacted.
This is a long recovery, a marathon not a sprint, and you have to learn to pace yourself.
Your PT will guide you about the use of a stationary cycle, starting first with rocking the pedals back and forth and gradually working up to doing a full rotation as your knee flexion increases.
It's important not to try to rush your recovery, as doing too much, too soon, can have an adverse effect.
Arm weights should be OK from quite early in recovery, as upper-body exercises shouldn't strain a new knee.
Here are a few articles, to give you some guidance:
the BoneSmart view on exercise
BoneSmart philosophy for sensible post op therapy
Activity progression for TKRs
Knee recovery - Lose the Work Ethic!!