Because they do - the single seats are REALLY prized by e.g. business travellers wanting to work in private. The lack of these in Pendolinos is a bit annoying, particularly as people are more "packed in" in First/Standard Premium than they were due to the reduced number of coaches - before, a table for two to yourself was near guaranteed.
Standard, interestingly, has a very high proportion of tables with just 72 seats per vehicle (if I recall from above) instead of the more usual 86. It all looks very "Coach D"!
Going off of the full livery on 805005 which handily shows the number of seats per carriage:
Coach A has 46 seats + 1 wheelchair, with 6 bays of 4
Coach B has 66 seats + some bike space, with 8 bays of 4 and a shop
Coach C has 80 seats, with 10 bays of 4
Coach D has 82 seats + some bike space, with 10 bays of 4 (presumably the two extra seats vice coach C are due to the cycle space)
Coach E has 25 seats + 1 wheelchair
(For reference, voyager coach D has 52 seats, with 12 bays of 4)
Here's the pretty much finalised version of the seating plan I posted a while ago (based off of interior references captured by me and also some videos from other people):