I know it is possible to sign up as a company to invest in Bitcoins on almost all exchanges where they sell Bitcoins.
But I am just wondering, what is preventing companies to invest big in Bitcoins and then pretend that someone stole their private key and stole the funds, but in reality they just send the coins and exchange them through a bunch of different currencies and Monero etc that is "impossible" to trace? Will there not be a lot of money lost this way? I mean hacks etc are happening all the time so how would someone prove they did not get hacked?
Are you asking why companies are not investing in bitcoins? Any company can invest in bitcoin if they want to, provided the shareholders really want that. I guess that's one of the first hurdles if it's a legit company, how do you manage to invest heavily in bitcoins. Unless the company simply manage bitcoin investments (like stockbrokers), most other companies would have a hard time justifying buying large amount of bitcoins from every exchange. Bitcoin has to be part of the business model.
As for them pretending to have their coins stolen, that's a bit more complicated but I suppose they can just hire hackers and try to launder it. The big if here is if you are willing to trust someone who stole your money to give it back to you.
Also why have a third party or an "insider" to the "hack". Why not just move the coins to another address, no one can prove who moved the coins anyway?
You look more legitimate if you claim to be "hacked" when you're having problems or you stole your customers' coins. Mt. Gox is a prime example of this, with "all your coins are safe" being the classic line. No one can prove what you did but if you haven't already provided some kind of explanation, the authorities will rapidly be on your ass.
Yup, true. Some unscrupulous people do this even for assets that are not cryptocurrencies. How many times have we heard of people purposely having their stuff stolen or destroyed for insurance money or to be able to use those without much regulation.