Eh, they exist. Not many people even know how to check the source or even if it's been verified. Saving bandwidth is definitely a reason. The EVM call to creat

Eh, they exist. Not many people even know how to check the source or even if it's been verified. Saving bandwidth is definitely a reason. The EVM call to create the contract charges a gas fee per byte. Currently etherscan and other chain explorer sites will recompile your provided source (and compiler options) and ensure your deployed bytecode matches their results. If so they publicly provide the source and a nice verification checkbox. Some also give you (the public) tools to make read/write API calls to the contract directly by introspecting the API.