Why doesn't the UK ask for travel bookings for a tourist visitor visa?

On top of the reasons Michael Hampton listed, the UK does not want people to purchase non-refundable tickets or hotel reservations and then find they've wasted large amounts of money if they are denied a visa, so they specifically advise against this, e.g.:

We take this opportunity to remind applicants that UKVI does not require a flight booking or tickets to be submitted with a visa application. Our online guidance clearly advises customers not to make payments or travel reservations until a visa decision has been received.

See also our previous question, UK.gov says do not attach “flight/hotel booking”, but then my auto-generated checklist asks me to tick whether I am attaching this document. The system does allow you to provide an indication of your travel plans, even without confirmed bookings. That answer quotes the guidance used by UKVI staff:

For visa applications, visitors are not required to provide an itinerary, but you should normally expect the applicant to have some plans for their stay, and provide information about this on the application form.

At the border, you should expect the applicant to be able to answer questions on what they plan to do

In other words, even without firm bookings, they still expect applicants to have some idea what they plan to do and how long they plan to do it for (five days of the usual tourist things in London or two months staying at the mother-in-laws house in Wales or four days at a conference in Edinburgh, etc...) and will assess the application accordingly.