The difference between the past simple ( I did) and the present perfect ( I've done) is - very simply - that the first tells us about an event in the past, while the second tells us something about the present.
For example, "I made a cake" tells what you did yesterday, last month or last year. There is no connection between this event and the present moment.
But if you say "I've made a cake", this is giving us information about the present moment. It's telling us that you have probably spent the last hour or so baking, and that there is now a delicious cake on the table ready for us to eat. Or perhaps it's telling us that there are no eggs left in the fridge because you've used them all. Whatever your reason is for using the present perfect, we know that your statement is telling us about the situation now.
I hope that helps.
The verb "to make" has many different uses, i.e., it is used in many different ways.
The simple answer to your question depends on the usage (meaning) of the verb.
"I made it" means you created, built, or constructed something and the project is over.
"I made it" can also mean that you were successful, e.g., Today, I finished/completed my university studies.
"I've made it" means that you finished/completed a journey or a project that you began in the past and now it is now successfully completed, e.g., My car broke down. I had to walk 10 kilometres and when I reached my destination I said "I've made it (to my goal).
''Why did you do that?'' / ''Why have you done that?'' are very similar, so either one can be used. The only small difference would be that the present perfect verb suggests that the thing you did took some time to do, i.e., it started in the past and is now complete.
Hope these explanations help a little...
I made it = I created it
I have made it = I have created it OR "I have succeeded in life"
You're having trouble with the perfect tenses in English. The perfect tenses always use have, has, or had as a helping verb. I think the tutorial below is pretty good at explaining when to use the simple past tense (I made it) versus when to use the present perfect tense (I have made it). The difference depends on what time frame you want to stress. (Remember that the verb is what tells the time in a sentence; in a way, verb tense means verb "time.")
