Because a phone running an unknown OS is significantly more dangerous than a phone that hasn't received security updates for years. For example, a malicious OS maker could add their own certificate to the root store, essentially allowing them to MitM all the traffic you send to the bank.
Liability works on the principle that "if it's good enough for Google, it's good enough for me." A bank cannot realistically vet every vendor, so they rely on the OS maker to do the heavy lifting.
Even if they wanted to trust a third-party OS, they would need to review them on a case-by-case basis. A hobbyist OS compiled by a random volunteer would almost certainly be rejected.
Liability works on the principle that "if it's good enough for Google, it's good enough for me." A bank cannot realistically vet every vendor, so they rely on the OS maker to do the heavy lifting.
Even if they wanted to trust a third-party OS, they would need to review them on a case-by-case basis. A hobbyist OS compiled by a random volunteer would almost certainly be rejected.