NPCs do not benefit from their ability modifier (D&D 5e)

Hello everyone ! I will be running my first ever game as a DM at the end of this week and I just found a potential problem : Every NPC sheet I use does not seem to be using the ability modifier when making an attack. I tried using sheets I created and sheets dragged from the compendium, but the problem remains. I then had the idea to try using the sheet from a high CR monster as a way to see if this changed anything and it did, the lich from the compendium does benefit from its +5 INT modifier. Is there something I'm missing maybe ? Do low CR monsters not benefit from their ability modifiers ? Thanks everyone in advance and sorry if this has been asked before, I could not find the right answer anywhere.

A good part of the point of the NPC sheet is that it doesn't have all the autocalculation active that the PC sheet does.  All those modifiers should be already built into the relevant rolls for creatures that are in the compendium. 

This is based directly on the 5e statblocks, which also have all of the modifiers baked in.

Ohhh so what you're saying is that the brown bear, in my case, has a +5 to hit which is just the sum of its ability modifier and its proficiency ? If so that is awesome !

Yup. All 5e statblocks follow this rule, with a very few exceptions. It has the added benefit of when transcribing to Roll20, the amount of data required is dramatically reduced.