I have no answer for anything involving English spelling or pronunciation. I will, however, point out that "desert" has two different pronunciations. It is often be a noun, for a dry, barren area where nothing grows--"the Sahara desert." With this meaning, pronounced totally differently from "dessert," a sweet course eaten at the end of a meal. However, "desert" has two other meanings. In these other meanings it is pronounced the same as "dessert." It can be a verb, "to desert," meaning to leave or abandon. It's often used for a soldier who illegally leaves the army. "Soldiers who desert from the army will be courtmartialed." It can also, rarely, be a noun for "that which is deserved." It's often found in the phrase "just deserts" (meaning "appropriate punishment.") In both of those meanings, it is pronounced the same as "dessert." Here's an unnatural sentence I invented, to show an example of "desert" and "dessert" being pronounced the same way: "Everyone should stay and help wash dishes. When we're done, we're having ice cream cones. Anyone who deserts their dishwashing duty will miss their dessert, which will be their just deserts." Here's one in which they are pronounced differently. "It's believed that sherbet was invented in ancient Persia. If so, you could call it 'the dessert of the desert.'"

thank you guys you did have helped me a lot :)

English has borrowed extensively from Anglo Norman French and Latin, so pronunciation does not always have uniformity.

The pronunciation IS different though. In American English. "Desert" is pronounced "ˈdɛzɜrt" and "dessert" is pronounced "dɪˈzɜrt". So both stress and pronunciation changes.

I don't think the e really sounds different, what is different is the stress given to the s: Des-ert vs De-sert. And there first e is a short e as in egg while the second er is said like the ir in dirt. See: https://www.macmillandictionary.com/us/dictionary/american/dessert And https://www.macmillandictionary.com/us/dictionary/american/dirt