This is a property of large, bloated legacy companies in any field. IT is still seen as a cost center and a secondary concern rather than the enabler of their business. As a result, pay/resources and "political capital" (for the lack of a better word) are allocated accordingly.
IT folks there aren't given the pay nor recognition they deserve, so no good talent joins or stays for long enough. Junior talent that joins ends up just learning from the mess and has no chance of actually becoming "good", so the problem continues.
Furthermore, the messy and unefficient IT systems benefit many people there, from lower-level menial positions whose jobs would be obsoleted by good IT to managerial positions who have a large list of reports to manage which gives them prestige and justifies their salary. Third-party suppliers also benefit as a bad IT system requires constant attention while a good system would require less attention (and a competent in-house team can attend do it, requiring no third-party involvement). Bad IT can also serve as cover - problems can be blamed on it instead of incompetence.
Fixing it incrementally from inside is politically impossible as people who rely on the status-quo will fight you on every step of the way. The only potential way is the organizational equivalent of a "full rewrite" - set up a subsidiary, give it unlimited money and task it with building a competing product. Operate it like a startup with the appropriate culture (especially regarding tech). Once the product is competitive, migrate customers onto it over time. This should be feasible at least for retail banking as UK fintech startups proved it's not actually impossible to create a bank from scratch. Rinse and repeat for every vertical of the business.