Why does every new project have to come out with their own token instead of just utilising existing tokens?

Every new project always comes out with a new token which acts as the governance token or "fuel" for the platform/network. Why can't they just use existing tokens though instead of coming out with their own token? For example, AMP is an ERC20 token, but even though ETH is already used for the gas fees, people have to buy and stake AMP to provide collateral for the Flexa network. Why? Why can't we just set dedicate some ETH to the network as collateral? We're paying ETH gas fees anyways so why does it need a separate token?

That was just an example, but there are countless examples as well for pretty much anything built on ETH, SOL, ADA, DOT, etc. Why not just use the coin the gas fees are paid in as fuel for the network instead of having to buy a separate token? For normal computer software and apps you don't have to convert to a separate currency for every single microtransaction, so why is this necessary in crypto?