Because FDIC only insures $250K? SIPC only covers $500K somewhere like Schwab[1] - or do you mean because you think it's terrible not be earning something on the $8M? Presumably that's enough for anyone to live out the rest of their life on comfortably...
I mostly meant for safety. You want it split up for many reasons... so you can have the FDIC or SIPC insurance, so you don't have to worry about losing access to the account via fraud, etc.
It would be ok for a short term to keep it in checking or savings accounts, but don't put it all in one.
FDIC is not just a dollar limit, but also per bank. Someone can have CDs from multiple banks, all in one account. Millions of FDIC insured money is easy these days.