PayPal do not care at all whether their APIs are developer-friendly. Why should they? They are a near-monopoly. You are completely dependent on them and they do not need you at all. I suspect their APIs are a result of minimising what they have to pay their own developers, because they win if they saved just one hour of development time even if 10 million vendors have to spend 10 hours each to work around the limitations.
In the very unlikely event that the developer has any influence in the choice of payment processor (i.e. you are a startup that intends to trade in only one country) they still do not care, because they still need to do the usual due diligence to check whether your company is a fraud/ML risk, but they earn little in fees from you. They would rather let a smaller payment processor deal with you, then try to take over later after your product is established.