Do I believe that aliens exist somewhere in the universe? Absolutely! There are more stars in the observable universe than there are grains of sand on the entire planet. And all those stars contain how many planets. And then you add all the moons of those planets. It is inconceivable that out of all those billions upon billions of options, the ONLY option for life is Earth.
However, do I believe that aliens have visited Earth, that UFOs have been here, and the government is covering it up? Absolutely not! Space is big! Really freaking big! The closest star system is about 4 light years away. At speeds we can travel, the fastest we can go with our technology, it would not take 4 years. No, it would take us 137,000 years to get to that closet star.
Any alien species that would be able to achieve interstellar travel, would be WAY more advanced than we are, and would almost certainly have no interest in Earth. We'd be insignificant ants to them, and I don't see them harvesting our resources as pretty much all of our resources, there's plenty more in space. Our oceans are nothing compared to the bodies of water in space (
https://www.fastcompany.com/1769468/scientists-discover-oldest-largest-body-water-existence-space). They can get far more metals from asteroid mining, and far more hydrocarbons from a planet such as Jupiter. I'd be shocked if aliens wanted to visit Earth for any reason. The closest I can think of would be some biological study in the same way we study ants. But even, it seems like it'd be too much of a hassle and not worth their time.