Why is ATP used as a source of energy rather than glucose?
Well the mechanics of this can actually get pretty complicated, depending on how far into the biochemical weeds we want to go, but there are a couple of very simple reasons at the core... Well they actually aren't that simple, but they can at least be analogized in simple ways.  The first, and easiest to illustrate is this.  This is a glucose molecule:



And This is an ATP molecule:



If you want to get some energy out of an ATP molecule, all you have to do is break off one of those phosphate groups.  If you want to add more, you just stick another phosphate group back on.  This can be done without significantly changing the structure of the molecule itself.  That Adenosine base never gets touched.  Only the chain of phosphates is manipulated.  It is much more energy efficient to add and remove those phosphate groups than to add and subtract elements from a glucose molecule, as there is no way to effectively break it down without significantly changing its structure, which makes it harder to build back up.  We want a molecule that it is just as easy to add energy to as it is to pull energy out.  The glucose molecule just doesn't have a chain sticking off that's easy to remove and put back at will.  The CH2OH group on the end looks like an attractive energy sink on its face but getting the whole thing off isn't nearly as easy as it looks, and it doesn't contain as much energy as a phosphate group either, which brings us to the second easy to illustrate reason ATP is more attractive than glucose as an energy carrier.

If you look at that chain of phosphates, you will notice that they are each separated by an oxygen atom.  There are also a bunch of convenient hydrogens and OH groups floating around that make hydrolysis and dehydration very easy.  Where would you hydrolize the glucose molecule to produce a low energy molecule that can be converted back to glucose more efficiently than ADP can be converted back to ATP?  Just compare this process:



To this one:



The ATP hydrolysis looks more like the breaking down of polysaccharides into glucose than the catabolism of glucose itself, and this is at the core of the difference between the two molecules.  It is always easier to add and subtract links from a chain than it is to make direct modifications to one of the links themselves.  Glucose is great for energy storage.  You can pack a lot of energy into a glucose molecule, but once you get it out, it's very hard to put it back.  It's much easier just to throw out the waste and start over by making or eating another one.  We don't have this luxury with ATP.  It has to be able to drop off its energy load and go pick up more.  For this, the structure of ATP is much more appropriate than the structure of glucose.