That's $2 per month of cost.
A developer costs $150k a year, and if it takes a month to develop, you could have the service for some 5000 months.
In-house developers should focus on the business domain, not on custom building business processes.
Legal needs to review because it is sending employee PII (emails, phone numbers, etc) to a third party, who now knows the individuals in critical "approval roles".
Next hit up security and have them do an audit since this is going to be part of a security control. For bonus points, the internal pentest team finds a bypass that ApproveAPI needs to fix.
Your $150k a year developer is now spending 3-5 hours a week for 3 weeks shepherding a vendor onboarding for something they could have built and tested in a few hours.
And in most cases, vendor management isn't going to get involved for something that will be expensed on a credit card for $2/mo
Once had an internal infrastructure team estimate £70K for the infrastructure to host a single static HTML page.... :-)
But it is part of a business critical flow, and therefore handing it over to a third party is absolutely unthinkable.