Technologically, nothing. They've even been made before! (Japanese scientists even made a device that would let you taste things!)

The problem is, nobody actually wants to buy them so nobody is making them for people to not buy because that would be a waste of time and money. Knowing that death and sewers are super common in games, I can't say I would want smell-o-vision myself.

https://en.m.wikipedia.org/wiki/Smell-O-Vision

Images flash by and disappear. Sounds may resonate a little but are basically gone as soon as you stop making them.

Smells linger.

Imagine the cattleyard smell still hanging in the air when the scene has changed to milady's boudoir, or to the fancy restaurant.

Too complex, too many inputs needed, it would be a proprietary exploitation product nightmare like liquid ink paper printers, initially bringing such a product to market would make it cost a fortune, and it would need widespread adoption before the economy of scale could kick in.

I saw a theatrical performace in 1997, called The Peplum by theatre company Royal de Luxe.

It included 'odorama', a contraption that looked like a TV studio camera on dolly rails across the front of the stage, that sprayed the audience stands with about 6 different smells throughout the performance.

One of the smells was horse manure, which was a nervous shock for the audience. Yes it smelt like horse poo.

A very memorable performance, because of the smells, but also the exceptional company behind it.

I remember a 3D show at Disney that did this when I was a kid. William Castle did something like that once too iirc

Edit: William Castle did the thing where he shocked viewers with electricity, not the smell stuff.

Edit 2: I think it was this: https://en.m.wikipedia.org/wiki/It%27s_Tough_to_Be_a_Bug!

They released a stinky smell when the stink bug character was on screen.

Mmm...styrene, benzene, and other carcinogenic aromatic hydrocarbons...

Smell is a pretty complex thing.

For vision, we only have four different kinds of receptors, which can be stimulated by electromagnetic waves on a one-dimensional spectrum.

For smells, we have about 350 different kinds of receptors. Also, they can't easily be stimulated by electromagnetic waves, but only by molecules, which are much more difficult/costly to transport to their corresponding receptors.

It's a mystery who just read Ready Player One. A complete mystery.