But that is the trivial part for. If it is trivial, if the PCs have no chance or only a minuscule chance, you don't let them roll at all. The DC Table in the DMG is for Task that have at least a 5% Chance of failure. That's how you need to read that chart. If it can fail, you use one of those DCs. If you think it is a task where the characters shouldn't fail, you don't make them roll for it.It has nothing to do with the "standard bonus" at first level because many people/creatures do ability checks that don't have "levels" or proficiency in a skill.
The DC 5 isn't "Very Easy" for 1st level PCs, it is very easy for every living creature.
Even if I agreed with your logic, should a 1st level PC not succeed at "very easy" task even 10% of the time??? IMO, no, certainly not.
A very easy task should only be failed if there is negative modifiers and likely disadvantage. Reducing the DC to 0 accomplishes that.