Each country and sometimes even states have different regulations around money and financial services. It is a complex and expensive landscape to navigate and takes a long time to get approval for license applications.
Building any product can be a difficult feat, but dealing with money and transactional systems introduces an additional level of complexity in terms of scalability, reliability and security. Not only do you risk failing in building an unsuccessful product, but it is possible to make significant mistakes that can result in the loss of money.
Innovation in finance has traditionally been left to large banks. Banks have laid the foundation in terms of payment networks, settlement networks, relationships with regulators, etc. It is not always easy to get in contact with the right people to get access to the banking and payment systems you need to build your business.
Traditionally financial business models are capital intensive and are only profitable on scale. You need sufficient runway to build a strong user base to be profitable.
It is hard to find talented and experienced engineers with a strong interest in building financial infrastructure. Not everyone is cut out for the job and you have to compete with banks that can pay large salaries for this type of skill set.