That's what the majority believes for Ethereum. While bitcoin is the king of all cryptocurrencies and the market then ETH is specifically the king of altcoins.
You may think of any other altcoins that's being considered as the king of alts but, the sole basis right now is the rankings that they get from websites like cmc that we used to visit and that's basing through their market capitalization.
I wonder what is that other altcoin who also has the title of king of alt? But if I can guess, it was BNB? Because this alt is backed by the number 1 exchange (Binance) and also has lots of supporters but if we look about the date of creation and other details, ETH can still win the competition.
People based on the rankings, that is because rankings does not lie and before CMC ranks a coin, they have done a lot of calculations and analysis. If the cryptos has gender I think it will sound better to call it a queen than king because ETH sounded like a feminine name. Don't worry because queen is still the same as king in terms of ranking. It was still number one.