Why don't people change supplier?

Because it's just not worth it.  People tend to look at changing supplier just after their prices have risen - after all that got to be the best time to look, right?  No, because all suppliers raise/lower their prices at different time.  So you're likely to switch to a supplier who hasn't raised the prices yet - so you expect to save £100's.  But unless you chose a fixed rate tariff (more expensive, and not worth the premium in many cases), the prices are likely to rise soon after you join - erasing all or some of the savings you've made.

So when should you look at pricing?  Once all the suppliers have raised thier prices - but that can take weeks, and do you have time to check that all suppliers have raised thier prices?

The simplest strategy is to check your prices on a regular basis, for example every 3 months.