It’s because a dictionary does not have to show conjugation. It’s purpose is to explain the meaning of the words and the correct spelling.

In French (I do not know for other languages) we differentiate spelling and grammar. If you want to see all the forms of a verb, you don’t want to look into a dictionary but into a "conjugueur" (most known is the "Bescherelle").

There are tons of websites where you can find all the forms of a verb. Here’s one with your example "être":

http://leconjugueur.lefigaro.fr/conjugaison/verbe/%EAtre.html

The conjugated forms are not needed when you have learnt the patterns and the exceptions.
In the mentioned case, only those forms that are exceptions from the normal pattern are printed, the remaining follow the rules.

For languages with many forms (French, Spanish, Latin, Greek, even Swedish! that uses one form/tense) there are special books on the market that are focused on listing all forms of verbs, e.g. “501 NNN Verbs: Fully Conjugated in All the Tenses” for a language. I have got some of these, but seldom use them for the reason above.
There are some dictionaries, though, that do include lists of the most common irregular verbs with forms.

A dictionary deals with the meaning, spelling/variants, pronunciation and the usage of each word, and sometimes etymology.

NB. There is a tendency to simplify the use of languages with many forms, e.g. substituting with simpler forms or dropping word parts. English has undergone some already (I have, thou hast, he hath…), as has many European languages. Interlingua is basically free from personal verb forms, and still understandable for French, Spanish and Italian speakers.