Because from experience I'd say it's not worth the effort. I can count on 1 finger the number of roles I think I've missed out on due to not tailoring my CV. I need two hands to count the roles I've got which have moved from CV submission to a request for interview within 24 hours.

By the time you've carefully read through the job spec and then altered and changed your CV appropriately, the agent will have 5 more CVs from other suitable candidates and will be sending those instead. The tailored CV ends up on the "B" pile in case the client doesn't like the first batch.

Obviously this isn't true for all roles which you will apply for, but it will be true for the majority and why miss out on opportunities just to get the perfect CV submitted which may never even be looked at by the hiring manager.

It really does come down to numbers and hard sales techniques.

But as always YMMV