Why do people often not use capital letters when writing modern English?

Traditionally, capital letters are used for the first letter of each sentence, and for all “proper nouns”, to indicate that a specific person place or thing is being written about.

I notice that for ten or more years, there seems to be a fad of not doing this, particularly with people’s names, pseudonyms, logins, and even the pronoun I,but also for names of nations, etc.

I am interested in people’s reasons for not doing this. I realize that there will be many different answers, and there is no wrong answer. Please share.