Because Starfleet makes their new ships with planned obsolescence.
Adm. Stolnayech: Oh for Cochrane's sake! There's that annoying Utopia Planitia salesman
again! I'm erm ... I cannot be disturbed, I urgently need to prepare for tomorrow's staff meeting. Captain Doves, surely I can entrust you with, erm, receiving him?
Capt. Doves: Sir, I really need to...
Adm. Stolnayech <barking>: That's an order, Doves!
Capt Doves <looks unhappy> Yessir.
Salesman: Good morning, good morning! Did you smell the fresh coffee this morning and wonder what exactly made it smell so good? Well, our new
Janeway class of vessels has been equipped with the latest in sensor technology, and it will tell you the answer in a picosecond!
Capt Doves: I don't drink .....
Salesman:
and she's 20 percent faster than the obsolete
intrepid class!
Capt Doves: But we only bought those five years ....
Salesman: faster warp jump initiation times too! Multi-Transvector-Assault-Mode, a huge upgrade from regular Multi-Vector Assault Mode! Don't even
ask me to describe the differences, because I haven't got the faintest clue! haha!
Capt Doves: Impressive, but we ...
Salesman: Not only does it come equipped with it
three EMH's - mark 7, no less!, it also has an EMO (Emergency Morale Officer hologram) pre-installed!
Salesman:
and you'll receive a fifteen-percent discount if you retire your old
Intrepids within three months! You
know we're cutting support on those 15 days from now so you won't be able to replace parts any longer, don't you? Here! Sign on the dotted line!
Capt Doves <sighs and signs>.