T-1000 could morph into anyone's body. Then, what was the point of using the police officer's body all the time when the T-1000 was chasing our heroes? What was the point of notifying them that this is me? Couldn't he simply walk near John Connor as a homeless beggar and finish his task?
John and Sarah, after the initial meeting in the mall when Arnie shows up, actually never interact with any strangers. They only speak to people that they know/ recognize, or that the people they know can vouch for. The T-1000 would not get the opportunity to get close to them in another form, they would never trust anyone because they know that any stranger could be the T-1000.
Plus, if the T-1000 was close enough to fool them with a disguise, he would be close enough to just run up and kill them with a gun.
The T-1000 probably knows this, so its entire strategy is to chase them relentlessly and kill them as soon as it gets close enough. He only changes form to fool other strangers, or to try to lure them into a trap (as is the case with John's foster mother).
Technically he didn't. He used the form of the security guard in the ward Sarah was held in. He also imitated Johns foster mom, and Sarah Connor herself near the end of the movie.
One can only guess he didn't use many more forms than that because:
The forms he took were essential to infiltrating or gathering information about his targets
To prevent confusion amongst viewers. If we spent the entire movie going "i bet thats him!" it would take away from the drama.
This referred to (at least tangentially) in the Frakes novelisation.
- The "Officer Austin" form is described as being "energy efficient". It's certainly possible that once T-1000 has assumed a base form, this is the most efficient form it can use:
The T-1000 walked forward. Because of the man’s obesity, it had been stressing its ability to expand molecularly by remaining in the Lewis form, so it had defaulted back to the more energy-efficient Officer Austin. It reached the bars. But it did not stop.
- The policeman disguise prevents him from being attacked by humans.
One of the cops who pushed back the crowd had a handsome young face and cold eyes. There are always cops at disasters and scenes of violence. The mercury man’s choice of protective mimicry was, of course, perfect. Perfect because it was here to protect and to serve.