Functionality is either needed or it isn't, but it doesn't need to come from an external dependency. When it does, it probably comes with functionality you didn't need too. And as soon as you have a compile/runtime dependency on external code, your compile/execution environment needs to always have access to third party code. So that's bloat and complexity. You also give up control. Hopefully it ends up saving a bunch of time over developing it internally.
Hopefully an upgrade to an external library doesn't end up including another dependancy that happens to include some backdoor that steals all the credit card information in your database. Or a crypto miner in frontend code. Or introduces a bug that stops people from being able to checkout. Or the money package starts calculating slightly differently than your payments provider... Etc. etc.
