It takes a lot more reasoning ability than a horse or a dog have, and those two are smarter than your typical animal.
It's because of the limitations of the game mechanics. The game only goes down to -5 and a basic animal is at -4. A monkey who is smarter than basic animals should be at -3. That puts apes at -2. You could more accurately reflect things if there were more penalties, such as if every number below 10 was a penalty of 1, instead of every other.
It's reasoning ability, which is explicitly intelligence in 5e. There might be a component of wisdom to it. It's wiser not to hit something hard with your fist 5 times before you reason out a solution, but the solution is reasoning. The hunting tactics of chimps and advanced tool use of apes came about from intelligence.
It's not the same kind of reasoning as spell casting, writing, and history though, is it? Why would apes with an int of six not easily be able to be taught what a random other humanoid with an int of six can do? If they can't do everything an arbitrarily chosen creature with an int of six could do, then do they have an int of six or a lower int with proficiency in tool using?
Hunting is. Tactics are not. What the Chimps do goes well beyond tracking down prey and killing it.
It does not take most animals long to figure out that biting or pawing at a porcupine is not a good strategy to repeat.
But it's the ones with a 6 int that go get a tool to successfully get that porcupine anyway.
