It's because those are critical phases of flight, with an especially high workload on the pilots. This is also why small talk between pilots is disallowed durin

It's because those are critical phases of flight, with an especially high workload on the pilots. This is also why small talk between pilots is disallowed during takeoffs and landings. If an instrument acts up in cruise, the pilots have a lot of time to figure things out, while an instrument acting during landing is much more likely to lead to dangerous situations.