Why can't we toast bread in a microwave?

Because the microwave does not produce infrared waves. These are for the 'toast' effect. At least in my experience, putting bread in the microwave results in either soggy bread or rock solid bread. It just can't get crispy without a heat source.

For getting the 'toast' effect, the temperature needs to be more than 150 deg Celsius. Microwaves produce heat due to rapid movement of water molecules, thereby maximum temperature can go only up to 100 deg celsius After that, there is only loss of moisture and bread becomes 'dry' rather than becoming 'crisp'