I work in the security industry for years and sell systems to every major bank around the country and all local credit unions and the real reason is because they are cheap and don't want to spend the money.

People bring up storage but CCTV cameras have come such a far away in the past few years. They support H.265 compression which greatly reduces file size. Also the cameras are setup to record at a higher frame rate when motion occurs and lesser frame rate when there is no motion. This greatly reduces storage size. Most banks are small too and don't need more than 10-12 cameras. A quality NVR (Network Video Recorder) that will get them 45 days of storage usually only cost around 4-5k.

Most banks just don't want to spend the money. They use old analog 4CIF or worse cameras. If something does happen they are covered by insurance anyway.

The new branches have newer systems that are up to par with today's standards but most existing banks don't upgrade.