Why can't we use 100% of the brain in a certain moment? I know it's a myth that we only use 10% of our brain, but the point is that we use less than 10% in any given moment; only by using different parts of our brain for various tasks over time do we ultimately use 100%.

Let's imagine that we have a GPU, a CPU, a quantum computer, and a biological computer that can work perfectly.  It's obvious that some of them are going to be better at performing some things than others, but the point is that anything that can be computed can be done by any of them.

I think that similar logic should apply within our brain: Although some areas of the brain might not be too good at processing some things, they could also work towards the result.

What is the reason that this doesn't happen?  I believe that it would be a significant improvement in the brain's performance, although it would be very energy consuming.
The widely quoted figure of "10% at a time" is actually overestimating simultaneous brain activity by up to an order of magnitude. As demonstrated by Lennie 2003 (Current Biology), the 


  number of neurons that can be substantially active concurrently


is possibly as small as 1% of the brain's neurons, due to the high metabolic cost of spiking. Generally, metabolic cost is a major determinant in brain activity. This in turn entails that even


  less than 10% of cortex could be active at all


, to quote Lennie again. This number, derived for humans, is lower than for other animals, who are capable of sustaining a larger fraction of their brain's neurons simultaneously. This means that rats and hamsters use more of their brains than humans do.  

Paradoxically, the highest metabolic cost comes from inhibitory activity. Also, neurons restrict their own firing rates via refractory periods. For the best example of a failure of this inhibitory control of excessive spiking, consider epileptic fits, which correspond to extreme synchronization of neural activity, in a sense an over activation of the brain.
I think considering that two examples of brains showing more concurrent activity than the healthy human brain are 1. the rat brain, 2. the epileptic brain, it becomes obvious that increasing total activity would not inherently improve brain function.