Your attendees book hotels the minute they register. Outside your block.

Usually the majority. Attendees have loyalty programmes, corporate travel policies, per-diem limits and preferences that your block cannot match, so a block that captures a large minority of attendance is doing well. The rest still stay near your venue, which is why pickup is a poor measure of the lodging your conference actually generates.

By not holding the inventory. A housing bureau exists because someone has to manage blocks, rooming lists, changes and attrition across several hotels. If the offer to attendees is bookable directly and the booking is handled behind it, there is no rooming list to manage and no clause to defend.

To booking sites, almost entirely. The attendee registers, opens a new tab, and pays a platform for a room within a mile of the hall you rented. The conference caused that transaction and earns nothing from it, which is the specific gap this is built to close.