Because instead of waiting for two years for an Atlas ride (or other/worse), less than 3 months for an opportunistic ride to orbit. Which, if the bet on reuse is successful, drops to a month in less than 2 years.
Not really feasible (3 month much more than 1 month).
a. The spacecraft has to be already built and sitting around (not in storage)
[NB will explain Jims rebuttal for those who don't understand what he's saying, then give my counter argument.]
Spacecraft aren't "timed to launch" in terms of manufacturing and qualification. It's more like separate works of art that are commissioned, consume an organization, and are pushed out, then there's a lull before the next,
Some, like the GPS constellations, have ground standbys/secondarys that are ready to go. Rare.
Counter argument to Jim - some like SES want to horn in on Boeing's territory, so they can actually do more than one concurrently.
b. There has to be a spacecraft crew has to be available.
SC have a retinue, like a hollywood star, that accompany them as they are processed into a payload, tested, encapsulated ... delays, things don't work, ... eventually they make it to the pad, launch, and they recycle (with time off).
Often you have additional people on said team or related from the SC manufacturer, so that if someone quits they don't take the business with them. So you can "do more" because you do have flex, but you can't overcommit and ever let down your customer. This is a problem if you have demand for the same thing supplied by same people/person at the same time.
Counterargument - the customer base for a early on orbit activation will cut you considerable slack because you are allowing them to earn millions quicker by not sitting on the ground. They will also "team" with SC operator to allow another customer to "share"/overlap a part of the team, because there will be more total "eyes" on their SC so they'll get better quality for the same service.
c. Spacecraft EGSE, MGSE and FGSE have to be available. (some spacecraft manufacturers have only one set of critical hardware)
EGSE, MGSE, FGSE = Electronic/Mechanical/Fueled Ground Systems Equipment
They have only one set of hardware because there's no need for more. This was true also of other rarified services that seldom had demand. Some of this equipment has copies in manufacturing facilities, especially handing/checkout/test equipment, because when you assemble/qualify a bus you need to work it in the same manner (although you'd have to outfit customer specific portions to them as well).
Counterargument: Are you nuts? So they outfit an existing handling fixture from the assembly floor, borrow a bring-up bench used for the proto bus, and add on the transponder checkout rig, while the assembly support team back fills with bringing up replacements for both in the time window to the next bird, all getting a bonus for overtime because they got ahead on the calendar year with an additional mission under their belt.
I'd want that bonus.
[The rest is a bit much IMHO. Think its obvious so will forgo being long winded.]
And no, fueled operations are limited to a very limited (thankfully) part of the processing just prior to mate/launch, and apart from operations congestion around a step in processing. That's going too far.
d. Not going to happen for a first flight of a new spacecraft. That will take at around 12 months for analytical integration.
Most sats are based on a few busses that have already flown and had the same integration. You're straining at gnats. And no, don't think that a NASA planetary mission or AF DMSP derivative is going to want to ride a reused booster any time soon

e. There are other non launch vehicle items that have to be performed and scheduled (tracking station reservations, FCC applications, etc)
For GSO launches, you could "swap" slots (if the same operator) and sign up for a later flight for a discount for one not urgent to fly.
f. 1 month is not going to happen because the spacecraft would have to be already at the launch site.
You can deliver two when you deliver one.
Atlas did Cygnus in less than 12 months
BFD they switched a booster between customers. ULA can do a fast launch too. What they can't do is build a LV in the time a booster can be reused, and a US built.
And, before another specious claim gets started:
g. risk of SC damage because concurrent operations with multiple SC on site overtax resources
No, we can (and do) handle concurrent payload operations, where some can become stalled/sidelined, where the issues can be worked while others proceed to launch. Even SC from the same vendor/operator Jim ...
Yes there are some paperwork/regulator issues. Just like before airlines swapped slots. I could get into the minutiae if you like ...
And we haven't even talked much about the case where the sucessive SC / launch ops are non conflicting.
Oh, and what would be the SX issue to handle this? Need more payload processing facilities. Gee, where would they get more space for that ...