Why did the commanders insist their men to hold on to grounds till last man, last round?

Answer 1

To inflict as many casualties as possible on the enemy. The enemy may win but it would be costly. Retreat can easily lead to panic which is contagious. A retreating army tends to cause panic in other areas.
A formation that holds on to a piece of ground forces an attacking enemy to devote time and men to reduce it that otherwise could have been exploiting a breakthrough elsewhere. So there is in some cases a method to the madness.

Answer 2

I suppose they wanted to go down in a blaze of glory. No commander wants to be known as the one who let his men retreat. They want to be known as the commander who perservered and won the battle at the very last moment, when all had been thought lost.

I feel sorry for the poor soldiers who had to listen to them, though!

Answer 3

Large inflated egos I suppose.