Why do people pronounce "th" wrong?

My friends and my teachers, everyone I know, pronounce 'th' not correctly. They say the 'd' sound for 'th,' but 'd' and 'th' do not sound the same. Why do people pronounce it wrong?

Because our people are not familiar with such pronunciations, which is understandable considering the nature of our language. And teachers generally tend not to care about these kinds of "details" in their classes, which are actually just basics. People learn it the wrong way and keep it that way until their mistakes are addressed, which happens quite seldom since generally no one cares.

The mistakes you make in pronunciation, like the punctuation and grammar errors in the previous version of text you wrote, are also part of the learning process. For non-native speakers like us, it is natural to mix up the 'th' and 'd' sounds. The important thing is whether you are understood or not. I've never asked anyone, "Hmm, you used 'd' instead of 'th'" or "You pronounced 'two' as 'too'. Can you repeat that?"

Consider this as part of the learning process.

This is fine. This is common to mispronounce some sounds if the sound doesn't exist in their native language. It leads to accents. Germans don't pronounce 'w' correct, they just say 'v' and they deliberately prefer this because they think that this is part of their German identity.

Likewise, let people speak. Not one have to speak like a British or American if they don't indent to integrate themselves into an Anglo-Saxon country.