Why is radioactive decay so rare?

[cmcraes]

Hi all, I'm just curious about the weak force and how it works.
If quarks and electrons both have weak iso-spin and are constantly whizzing about next to their neighbour's (especially so inside a nucleon), what prevents say, the up quarks from emitting a W- boson everytime they are near a down quark?
Thank you!

 

[A. Neumaier]

A W-boson is quite heavy and can be created only when a lot of energy is available.