It amuses me that browsers in incognito mode refuse to allow screenshots on mobile. But same browser running incognito on a desktop can be merrily screenshotted. What is the difference they are trying to enforce based purely on device form factor/OS.
My pet theory: it's because Snapchat got big early, platforms added the feature to facilitate Snapchat's business model, and then banks started abusing it, and it stuck around "because sekhurity".