IANABiologist, but from what I understand there are two kinds of muscles.
Slow twitch muscle is what we have a lot of.
And fast twitch muscle is what, like, chimps and gorillas are made out of.
And pound for pound fast twitch muscle is a hell of a lot stronger than slow twitch.
But slow twitch is capable of far, far more fine control. It’s the reason we can type and write and manipulate small objects with a lot of precision. A Chimp can tear your arm off because his fast twitch muscles have a lot of power, but he struggles to manipulate small fine objects. You, on the other hand, can type like nobodies business but you would have to lawyer up, hit the gym, and delete face book for a long time before you could tear anything off of anyone.
To add a little more detail; it has to do with the number of muscle fibers activated when an action is preformed.
From what I understand apes and chimps by default activate a large amount of muscle fibers at once and humans can activate a smaller number. The number of muscle fibers activated is what gives apes and monkeys an insane str/size ratio.
As FrankManic said activating large numbers of fibers has the trade off of precision. It’s hard to activate a lot of muscle fibers for short periods of time.
Interestingly enough adreneline gives humans the ability to contract larger numbers of muscle fibers at once. Ever heard of someone lifting a car off their child, or showing some feat of super human strength?
The down side is you lose precision when you have adreneline in your system, next time you get a huge scare or shock of adreneline and you feel your heart pounding, try to sign your name on a piece of paper. It should be harder or messier than normal for most people.
We also lost a lot of muscular strength so that our jaw muscles wouldn’t interfere with our large skull development.
All human muscle fibers have an all or nothing response once activated. Whether we or a chimpanzee is doing a fine or gross motor skill depends on the task at hand… Gorillas can be gentle too, how else would they handle their young? Animals like gorillas and chimpanzees have more fast twitch muscle fibers than humans, which are the kind of muscle cells that allow for strength and power. Not only do they have more muscle pound for pound, but they are biomechanically set up to deliver more strength in specific ways (swinging from branches, climbing trees, certain attacks that it may use to survive). humans are weak and slow compared to most animals, but we can outrun almost any animal for distance, especially in the sun.
Source: Masters in Exercise Science
The basic answer is because we (and they) evolved that way. Remember the evolution starts as something random.
Take a bird. One day a bird is born with a longer beak. This longer beak allows the bird to reach deeper into the ground for food. Over time that bird has kids and they have a longer beak. Then their kids have a long beak. Eventually there is a drought and there are no worm to be found near the surface of the ground. Only birds with long beaks can get food. So the short beaked birds die and the long beaked birds survive.
Something similar happened with humans a long time ago. Instead of evolving to be stronger or hairier we evolved to be smarter. Being smarter meant we don’t need brute strength to survive. Instead of being strong or fast or having claws we learned how to make spears. Instead of growing longer hair or fur we learned to make clothes. So strength, agility, or other features weren’t necessary.
Other animals didn’t evolve to be smarter. They evolved in a different way to survive. Dogs evolved for speed and agility. They also evolved to have fangs, claws, and fur. This allows them to survive in the cold and hunt prey.
Apes and monkeys took a different path than us. They didn’t evolve to be very smart. They evolved to have more strength and less brains. This allowed them to survive challenges that they faced.
Very few animals that are smaller than humans are actually stronger. Some are, such as apes or monkeys. Physically this can be explained by their physiology, or how their body is made up. They contain a different ratio of kinds of muscles that allow them to “burst” strength better (but at the cost of being able to be strong for long). Their muscles also attach to their bones differently, giving them more strength at the cost of fine muscle control. Other animals, such as dogs, are not stronger than humans. They can appear that way as when they’re trying to do something that they’re good at. Consider how much a human can carry compared to the animal in question. Humans can carry over their body weight, if properly attached to themselves, for quite a long time. Other animals will struggle.
TLDR: Humans evolved for intelligence rather than raw physical capabilities. We also evolved for fine muscle control and stamina over brute strength. Very few smaller animals are stronger than humans.