It's theft of labor - which is exactly the same reason stealing anything is illegal. There is no fundamental difference.
Your labor can produce a value - car or product. Stealing the car deprives you of its value, the value of your labor that you converted into the car. That's fundamentally why stealing the car is theft. Stealing your research deprives you of its value, labor that you've converted into a valued product. It is exactly the same.
Not really.
If someone steals your car, it's gone. Value gone, all gone. You get nothing.
If someone steals your research, then...it depends what they do with it. If they convert your research into an identical product and totally swamp yours, then you've lost everything. If they do nothing, you've lost nothing. In reality, it's probably somewhere in between, you'll get some value from your research but not as much as you might otherwise have done.
The difference being that there is an additional step. Simply taking the information isn't enough. Someone has to convert that into a product and compete with you directly, and depending on the research that's probably a non-trivial process.
That's why stealing an object and "stealing" an idea are not the same. The object is gone, simply by definition of someone else possessing it. The idea is not removed just because someone else possesses it.
For ideas/research, I consider that the thief *gained* value at a lower cost than the original researcher. The original researcher still possesses the same things that he always has.
Until the thief puts himself in direct competition with the original researcher, there's no real reason to feel that the original researcher has been hard done by. If the researcher is denied product sales, or job opportunities because the thief has got there first, then sure. But those are separate actions in addition to the theft.
