Some mfr's have the tranny go into lockup early.... It requires a more beefy system, but they exist... I remember my '93 SW2 did it
As for why not? In slower city traffic, going into lockup and then unlocking and then locking up again isn't so great in the heat, wear and tear departments. But when the roads are more open, and you're not constantly taking your foot off the pedal (and unlocking), it's not a benefit at all Lockup on hills isn't so fun either -- especially when there's not enough throttle to change gears (finding holes in logic programming) Additionally, a locked TC won't give you any torque multiplication
When I was driving an auto -- I got up to my lockup speed as fast (reasonably) as I could and then let off in the throttle so it would finally go into lockup... It is like another gear (shift wise).
Exactly where I'm at right now -- especially with the hill logic programming. 35 mph is full lockup (25-35mph) offers partial lockup on hills. Peak mentioned something I notice as well -- full lockup is tough on FE at <2000 RPM, but after the momentum is going, it's great.
The service manual mentions the ECU altering the timing just before a shift, to make it smoother. I can see the action on the SG -- shifts, drops MPG for a moment -- but FE increases.
But yeah, if I see no obstacles for a while, I get up to lockup speed as quickly as <3000 RPM will allow, lock-up, back off the throttle and modulate. It takes a lot of lifting the throttle at the target RPM, and feeding it back (but not too much to downshift -- that drives ya nuts).