Why do 'get' and 'have' work similarly in
I got/had my car repaired.
but differently – that is are not complemented in the same way although they still mean the same – in
I got someone to repair my car.
and
I had someone repair my car.
Why do 'get' and 'have' work similarly in
I got/had my car repaired.
but differently – that is are not complemented in the same way although they still mean the same – in
I got someone to repair my car.
and
I had someone repair my car.
The verbs 'get' and 'have' are not completed in the same way, whether in an active sentence or in a passive one: it only looks as if they work the same way in a passive phrase like 'get/have something done', only looks.
Get:
Have:
So it is only the simplified sentences which give the – mistaken – impression that 'get' and 'have' work the same way. Of course, the middle sentence in each case, even if it can be re-constructed, is not used.
Simplifications 'complicate' the task of learners of English, because they make it look as if something one has just learnt does not apply – here, 'get sb to do sth' but 'have sb do sth'- as if English were an endless list of exceptions rather than a language obeying a large but limited set of rules one can hope to learn!
In the second case, the verbs are being used as causative verbs. Normally, causative verbs are followed by the infinitive, but there are 3 exceptions: have, make, and let.
In the first case, the verbs are not causative -- instead, they are being used as normal transitive verbs, with the object being the phrase "my car repaired", which serves as a reduced version of the noun phrase "repairs, which were performed on my car".
Another interesting point is that the verbs are more functionally similar in the second case than in the first. In the first case, the meaning of "got" has some overlap with that of "received". So we can say "I got a message", but we cannot say *"I had a message".