Why does Scar blame Simba with no proof while pretending to be innocent?

1. To put pressure on an already emotional Simba and force him to take the blame for his fathers death.

   By saying

   "Simba, what have you done?"

   He is forcing Simba to be the scapegoat.

   If he had asked "Simba, what happened?" or "what's wrong", Simba might respond in a calm manner he might have cleared his mind slightly allowing him to think straight.

   At the point where he asks this question Simba is traumatized after watching his father die.

   Scar then takes advantage of this already emotional Simba and asks him directly Simba, what have YOU done?

2. To quickly get rid of suspicion

   You must remember Scar just killed the King. If you had done something this bad you would want to get rid of any suspicion of you being the culprit.

   Scar is cold-hearted but he would never be able to be king if anyone even suspected he was involved.

   He quickly points the finger on Simba before he could even say anything.

   Also as a side note:

3. Its an effect used to raise tension and suspense.

   Obviously we as the viewers know whats happened and therefore it creates drama and tension when Scar asks that question.

   The viewers are already emotional from seeing Mufasa die but after seeing his son get the blame for it makes the viewers hate Scar even more and then heighten their emotions further, making it a really sad moment in film history.

Also note you are making a false assumption by saying Simba felt guilty because he thinks he caused the stampede by growling at a lizard.

The real reason for him to feel guilty is because he was in the ravine when he shouldn't have been.

Although you ask the question as per Scar's perspective, and have another answer which addresses that, you also mention Simba and his perspective.

Simba didn't consider all those things which you elucidate. Quite simple.

In his mind, he was responsible for Mufasa's death. Ergo, he had no reason to doubt that Scar would be wrong — or even to doubt how Scar would've known.
As the aforementioned other answer explained, Simba is emotionally traumatized at this point and probably not thinking clearly or thoroughly. He also has no reason to distrust Scar at this point in the story, and so he simply takes it as a given that Scar would know things exactly as he (Simba) himself would know. Indeed, Simba continues to blame himself and doesn't even suspect that Scar had a hand — er, paw — in Mufasa's death until Scar actually confesses.
It is a storytelling trope that children often expect others to have the same priorities and concerns as they themselves do. The ability to think of other people as having their own perspectives on the world is one which comes with experience, maturity, and wisdom; Simba didn't gain these until later.

If you walked into a room with 2 people in it, and one had been murdered, it wouldn't be an unreasonable assumption that the person still standing had something to do with it.

Scar walks up to a this exact scene: Simba and Mufasa are by themselves, and Mufasa has been killed.

So I don't think it's unreasonable for Scar to play the part of someone who had jumped to that conclusion. He doesn't need proof - the fact that Simba is the only other creature there is proof enough.