Because the UK ID card will be a massive database used and cross-referenced by many, many other agencies. Because the UK has a proven track record of leaving said data on trains or roundabouts. Because this data is controlled/used by self-important jobsworths whose arrogance is matched only by their inefficiency/stupidity. Because mistakes in similar (smaller) databases have already proved impossible to correct in a timely fashion. Because any UK national under the age of 40 is genetically unable to spell properly or consistently, so this will find its way onto the database as corruptions. Because any criminal who hacks into the system then automatically has access to every corner of your life. Because these same jobsworths have just made overfilling your bin (which is only emptied once a fortnight) a criminal offence, which will eventually have a negative impact on your ID and therefore your ability to fly an aircraft. These are just the points that spring immediately to my mind.
We already have photo driving licences which fulfill 90% of the security requirement, and could be made to 99.9% - but that wouldn't create enough jobs for said jobsworths.
Apart from the above, I have few objections to IDs ...