Skills Bonus for Monsters: effect of abilities on total

[seasthyday]

Everyone:
I am trying to figure out the correct way to apply a bonus to a Skill check for Monsters in general.
In the PFSRD, if you look at a sample creature like the Bugbear, it shows Skill Bonuses for the following:
- Intimidate +7
- Perception +8
- Stealth +10

I understand how those total modifiers for the listed skills are calculated; but what about the other Skills for which no Bonus is listed? For example:
- Escape Artist
- Swim
- Etc.

There are circumstances where the Creature needs to overcome an obstacle, like getting untangled from a net thrown over the Bugbear, or needing to swim across a pond.

Escape Artist is DEX based, so the Ability Score should provide a modifier of +1, and Swim is STR based, so the Ability Score should provide a modifier of +3, but no such Bonuses are listed in the PFSRD.

Do Ability Scores not provide a modifier for Creatures unless the Skill is specifically listed in the Creature stats? I don't get it.

Links for reference:
- Bugbear: https://www.d20pfsrd.com/bestiary/monster-listings/humanoids/bugbear/

[Bob Bob Bob]

It's just to save space and ink on the printing. They generally only list skills that the monster has ranks or bonuses in. Everything can do any skill (except the ones requiring trained only). You are right on the ability mod for the Bugbear but keep in mind it's wearing leather armor and using a shield, that comes with an ACP of -1 (the shield) for an end mod of +0 and +2, respectively.