Event triggers are difficult to support using the UI since every event has it’s own different data structure. Consider the difference between a timer finished event vs a zone enter event vs a button double tap event. Each one has different data that it reports in it’s own different data structure. So every event type trigger would need it’s own UI constructed.
Device triggers on the other hand are built on top of state and numeric state triggers which have a rigid structure and therefore it is easy to create a “one UI suits all”.
BTW device triggers have a lot of disadvantages and it is just as easy to use a state or numeric state trigger in the UI: https://community.home-assistant.io/t/why-and-how-to-avoid-device-ids-in-automations-and-scripts/605517