How does ransomware get into major networks, such as schools or other large public agencies?

I read an article about ransomware affecting the public transportation service in Kansas, and I wanted to ask how this can happen. Wikipedia says these are "are typically carried out using a Trojan, entering a system through, for example, a malicious attachment, embedded link in a phishing email, or a vulnerability in a network service," but how? Wouldn't someone still have to deliberately click a malicious link to install it? Wouldn't anyone working for such an agency be educated enough about these threats not to do so?

I wanted to ask in that community, but I was afraid this is such a basic question that I felt foolish posting it there. Does anyone know the exact process by which this typically can happen? I've seen how scammers can do this to individuals with low tech literacy by watching Kitboga, but what about these big agencies?

Edit: After reading some of the responses, it's made me realize why IT often wants to heavily restrict what you can do on a work PC, which is frustrating from an end user perspective, but if people are just clicking links in emails and not following basic internet safety, then damn.