Anyone know why I can't merge Sales Tax Payable GL accounts? are these native accounts and not able to be merged into 1?

you have those sales tax accounts because you have a nexus in this jurisdiction, at least that’s what NS thinks; that is when you add address to a subsidiary NS creates tax accounts associated with this subsidiary. No, you should not be merging them and it is not really possible, but you can disable them under Set > Accounting > Tax Control Accounts

Our accounting team managed tax nexus outside NetSuite for whatever reason. By disabling them they can still create a custom Sales Tax Liability account?

well, there is functionality attached to these accounts for reporting, payments, filings, etc. I would test all possible transactions in sandbox and see which native accounts did not participate; then you can disable those.