That's a short term view. Any system you build inhouse has to be maintained until you replace it, and often the longer it remains in place the harder it is to do that. You might save a small amount of cash (which might be important at the time tbf) but you're creating a major headache for later. Legacy code is debt, and that includes all your code. It's also a huge problem if the maintainer leaves because typically those small systems are owned by an individual dev who set it up in the first place.

Everyone who founds a company needs to remember that they're building a system of systems that all interact and influence each other, and you have to balance short term cash flow against long term strategy.