Oxygen content in the air. They don't have developed circulatory system and uses diffusion to get their cells oxygenated. Bigger the body, more complex the network of oxygen transfer needs to be and they cannot support their body with current oxygen contents in the air
The cool answer is the Square-Cubed law.
Think of it as this. We hypothetically discovered a cube shaped insect. It's 2 cm tall by 2 cm wide by 2 cm long. It's surface area would be 2 cm times 2 cm multiplied by 6 since it has six sides.
**A = 6(2cm*2cm)**
**A = 24 cm²**
It's volume would be 2 cm cubed.
**V = 2^3**
**V = 8 cm^3**
The ratio of area to volume is **24:8** or **3:1**.
High surface area is great! The only way for oxygen to enter cells is through diffusion from the environment directly to the cell. Larger animals have circulatory systems to help bypass this limitation. Blood acts as the middle man carrying food and oxygen to other cells in your body. Also the blood vessels add surface area on the inside of your body, but enough about blood vessels, we want to see this gigantify this bug.
Now, let's ask your question. Let's make this bug the size of a car and see what happens. Let's make is a smallish cube car. 4.0m or 400 cm for each side. Plugging it back into the equations give you:
**A = 6(400 cm * 400 cm)**
**A = 960,000 cm²**
Pay attention to what happens to the volume.
**V = 64,000,000 cm^3**
The area to volume ratio is now **1:66.666 repeating**.
As you increase in size, the surface area of your bug won't be able to sustain the metabolic needs of its volume. And this just keeps getting worse the larger you get.
The skeleton needs to support more weight, otherwise it just collapses from its own weight, so the obvious answer is to make the exoskeleton thicker and heavier, which then makes it harder for oxygen to enter and makes it harder to move it's huge bulky legs. Then since the skeleton is heavier, you need more muscles to move the skeleton, but then you need more muscles to move the extra muscles you've added and this just goes on and on.
There are a few ways to bypass the square cubed law. Aquatic creatures don't need to worry about fighting gravity as much as we do since increases in ~~mass~~ volume add buoyancy. That's why whales don't crush themselves by their own massive weight. And as the other redditors have mentioned, the amount of oxygen in the environment is definitely a limiting factor in the size of your bugs.
Hmm. Lots of answers that (quite correctly) explain that a beetle the size of a VW couldn't breathe. Not a lot of answers that address the exoskeleton itself.
Growing is expensive. Growing with an exoskeleton is more expensive. Our endoskeletons are cheaper, at least in part because we don't have to throw them away and start over.
Most creatures with exoskeletons molt. They grow in spurts, shedding their old skeletons when the beginnings of their new skeletons are still soft and stretchy. This comes at the opportunity cost of being more vulnerable than usual while waiting for the new skeleton to harden, as well as the metabolic cost of getting the new skeleton to harden.
There are other possibilities, of course. You're carrying an example around inside your head in the same way that turtles carry one on their backs. Instead of throwing away a whole skull or a shell, you can have pieces of it grow separately. There's no physical reason that the same method would fail, if applied to an entire exoskeleton.
There doesn't seem to be anything in the laws of physics that prevents, say, beetles from eventually evolving into car-sized descendants. However, the number of changes that have to occur to support that kind of growth -- developing a way to breathe at that size, to support its body weight at that size, to grow its skeleton to that size, to manage its metabolism at that size, and so on -- with every tiny, independent change in its genes along the way producing a viable and successful creature that passes its genes to the next generation, is more than enough to boggle my poor little mind. The laws of physics might shrug and say "meh, why not?", but the laws of statistics say "don't hold your breath".
And, of course, if the *only* thing that changes is the beetle's size, and on that drastic a scale, then the laws of physics whips out its tricorder and declares "it's dead, Jim -- split open like a rotten pumpkin".
They breathe through spiracles tiny holes in the abdomen and this only enables the oxygen to diffuse a short distance along the body, so you could have a long thin insect like a stick insect but it couldn't be that wide - https://www.youtube.com/watch?v=a7OPV3QZWfs