There are a lot of reasons:
For many people the sims works as a way to escape real life, introducing multiplayer would invite people from real life into your world with your sims, your relationships etc.
A lot of features that people want with the sims, personalities, more relationships, gameplay with depth, open world, color wheel, customization etc. would most likely not be possible to do online or these features would be heavily restricted. A lot of people suspect that the reason why the sims4 feels so limited is because it was originally designed to be online.
Mods would not work or not be allowed online, especially not mods that do not follow the recommended age rating. Some of the most popular mods for the sims4 are script mods which would be considered hacking in an online environment.
Time would not work like in the other sims games, there would be no way to speed up time.
EA have a history of implementing microtransactions in their games, especially their sports games, often by implementing roadblocks which forces you to grind for hours. If you want to see what that looks like, download the sims online on mobile, even the different hairstyles are locked and requires money to unlock that you have to work for in the game for a long time or buy for real money.
If the game would be online, they would most likely not be able to sell so many packs that they do for the sims, because it is really difficult to have an online environment where the world would look different depending on which packs the players have bought. Rockstar Games solved it in GTA5 online by making their expansions available for everyone, but you have to buy properties that cost loads of money in game so you either have to grind money for days or buy virtual in-game money.
Hacking could very well be an issue, and with the exception of a reporting system that they have on the gallery, Maxis does not seem to have any experience in security.
When a lot of things are happening at once in the sims4, you can experience something like simulation lag, which is the game stopping everything until some processes have finished, this is something that requires Maxis to go back and try to optimize the code to prevent this. Which is really difficult to do and is something that would be even more difficult if the game would be online.
When a player has a bad connection to an online environment the other players often experience network lag where they can experience things lagging or loading slowly. You can handle this by using good netcoding. However, netcoding is really difficult to do. Network lag will most likely be an issue.
