This is misguided thinking. You are less likely to hear about on-prem breaches in the news just because they are less likely to be newsworthy incidents, but they are even more likely to happen. Part of the reason people moved to the cloud is because it is much easier to have a team manage updates and security at scale.
Being a small target is not helpful these days, attacks on on-prem software today simply scans the internet, or uses databases by people who have already scanned it, and is automated.
On-prem software has not been spared from the recent spat of security incidents.
There is a difference of the security of software running in your cloud resources rented from AWS etc. managed by you and being a tenant in many SaaS systems all cobbled together all running on rented hardware. There is a lot more attack surface and a lot more diffusion of responsibility.