The reason it makes sense to treat those things as if they were a natural person under the law is because it allows for natural people to group their common interest into a single legal entity that is separate from any individual member.
So, take the lake eerie thing. Sure, you could write a law to protect the lake that focuses on natural people and corporations, but if something happens, say an oil spill or algae bloom, then each individual who thinks they are be affected by it would be bringing suit. in the case of a lake oil spill that's possibly hundreds of separate lawsuits for the exact same incident.
Or, the lake could be granted juridical personhood and the law making the same protections could be written based on that personhood. If the same theoretical oil spill or algae bloom happened, there'd only be a single case instead of quite possibly millions.
And that doesn't even begin to affect issues of standing and actual harm. If I'm some poor schmuck getting water from downstream of Lake Eerie and there's an algae bloom or oil spill, it's going to have to be massive for me to be able to prove i've actually been harmed by the spill and that the harm is enough to justify the costs of hiring a lawyer. Otherwise whoever caused the spill is going to either get the case dismissed because i can't prove i was actually harmed because of their spill, or they're going to argue that my actual harm is something like 86 cents because that's how much extra it costs for water filtration, so even with treble damages I'm entitled to less than the price of a large drip coffee.
And even if you think, well, individuals shouldn't be bringing any sort of suit for something like that, it should instead be via the government, the government itself is just another instance of a non-natural person.