If tarot was first used as playing cards, why then...

do the cards have esoteric meaning? why do we have the major arcana? wouldn't the original maker of the tarot know what they are doing then?

The thematic content of the Tarot trumps is derived from the Catholicism of the time. The cards would later be interpreted in the context of esotericism. The terms "Major Arcana" and "Minor Arcana" were first coined by occult writer Paul Christian and were not used by the original 15th century card players. The oldest known name for the cards was "Trionfi" meaning "Triumphs" The English word "trumps" is derived from "Trionfi" Triumphal processions similar to the ones displayed by Petrarch could be said to be the original theme of this game. I believe the symbolism played more of a role in the original Tarocchi game because the earliest such decks known as the Visconti Sforza Tarocchi not only lacked inscribed names they were also without numbers so the players had to memorize the sequence of the trump cards.

As a traditional game, Tarot is ahead of its time for having thematic content.
Let us compare the Tarot game with the traditional board game Snakes and Ladders.
Snakes and Ladders is a board game with the thematic content of Hinduism and Buddhism. It's like a Dharmic version of Candyland.
Tarot also has a theme; Triumphal processions of Western Christianity.

From a game players perspective, Tarocchi and similar card games are superior as games than Snakes and Ladders as there are actual decisions to be made in the games. Like Candyland, Snakes and Ladders is completely governed by the role of the dice. These board games are more suited for children than for adults.

Someone added the majors to create a new card game called Triumphs, but how and when this came about exactly is the $64,000 question. The illustrations on the 22 "trump" cards derived from everyday themes that would have probably been very familiar to most people at the time.

I think the first attachment of the trumps to the occult is attributed to Antoine Court de Gébelin in the late 18th century, but I'm not positive about this, it may have been Etteilla. Someone more informed may know for sure.