A president, as well as being head of government like a PM, is also head of state, and as such has a higher level of security (as the British queen does). And the recent (ie since Kennedy) attempts on the lives of US presidents have made their security tighter still, and is basically set up to prevent the president going beyond a certain level of risk unless it's truly vital.

Johnson is no coward, however, and he decided this was worth it, as a show of support, which also makes him look good. With the Russian forces withdrawn beyond artillery range, an unannounced visit to Kyiv during the day (when, I think, air raids are rare) was within reasonable bounds.