Why do so many bathroom sinks no longer have overflow drain holes?

Prime time for the wet bandits to make a comeback.

Are they meant for "wet rooms" where the room is the overflow? ie there's a drain in the floor.

Good god what about tubs?? I have a memory when I was a kid of filling a tub with a suddenly spinning faucet knob and ‘rapidly’ filling tub by the time I got a neighbor to help. I did learn about water mains that summer but still.

Sinkflation

It's cheaper, lighter, and "looks" better, without the overflow hole.

Looking at the local hw stores in my country more than 90% have overflow drain holes. Ther are a few designer faucets without that.

Funny, I work with someone whose four year old flooded an upstairs bathroom. She was supposed to wash her hands, played with the stopper, forgot to turn off the water, and nobody noticed until the water was dripping downstairs. 

No overflow on that sink.

If you have an IKEA in your area (or within reasonable driving distance), I had a complete gut of my bathroom done a few years ago, and had a terrible time finding a sink that was (a) not horrendously overpriced, (b) stupidly designed, and (c) that would actually fit in my (kinda small, it's a 100+yo house) bathroom.

IKEA won, hands down, in all three categories. 4 years later, it's the best bathroom sink I've ever used. And yes, it has an overflow drain hole.

The faucet and vanity are also from IKEA, and I really like those as well.

I get people are saying they have kids and pets and whatever but the chance of me overflowing my bathroom sink and flooding my house is basically zero.  I don’t even care about having a plug in it as I never do that.  Would be totally fine with just a no overflow basin.  Everybody’s needs are different.  I despise double kitchen sinks.  I know people where it’s a non-negotiable need.

Many sinks don’t these days. Some style sinks simply can’t. I just renovated my powder room with a glass vessel sink, no overflow.

When I was renting, we stayed at a house that had a super fancy bathtub in the main bedroom and we discovered the first time that our kid was bathed in there that they didn’t hook up the overflow drain hole to anything. My wife came running into the room telling me water was dripping from the can lights downstairs!

The owner’s solution was to put tape over the overflow hole (it was a small amount of water splashing/sloshing into it that was the problem, not too much water in the tub)

Smell. Those overflow drains grow smelly slime that you can't clean.

My understanding is this is a USA trend. Overflows are required in Canadian sinks

I don’t think people are filling bathroom sinks as often as in the past so for most an overflow isn’t really necessary.

As mentioned they make a great home for flies and other insects that like moisture.

*porcelator

It's form over functionality. Manufacturers are catering to the consumer need for more 'modern' 'sleek' looking bathrooms. It is frequently used as an aesthetic selling point. They are maginally easier to fit since you dont have to connect the overflow up.
It's also easier to mould and QC a shape without an additional hole in it.
Technically retailers also make more money indirectly through replacements for water-damaged vanity units, flooring etc. As obviously overfilling your sink isnt damage covered under any kind of warranty.
Overfilling bathroom sinks isnt on peoples minds these days I guess.
You're correct in your intuition though, an overflow on your sink can potentially save you a lot of money.

New kitchen sinks are adding overflows but this is fairly new

I just got a tiny no-overflow sink myself and have been racking my brain to remember an instance when any bathroom drain in any dwelling I've lived in has ever been left on unattended or accidentally turned on unknowingly, to a point where the overflow prevented flooding. I cannot remember a single instance of this happening.

That said if it's a concern for you you could mitigate it with a water leak sensor strategically placed wherever the water first starts to spill out when overfilled.

The overflow typically goes down the normal drain hole, just internally rather than from above. If the trap is clogged, the overflow will usually fail at the same time the regular drain does.

The only really practical use-case for a sink overflow is when it's been intentionally blocked (that may not mean "desired" when done by kids or cats), e.g. by a stopper, towel, or cat butt. In that case, the overflow will work as intended. But in those cases, kids need a spanking, cats need to stay off the counter, and guys soaking parts in degreaser need a separate shop sink.

I noticed that in my apartment when i moved in, which had a new bathroom sink without one, and thought it was wierd. I guess it's like others have said, between cost-cutting & mildew/cleaning prevention.

Plus the rise of low flow/water-saving faucets & aerators will make it take alot longer to fill the sink that full, compared to having no aerator or standard aerators installed. (I have mixed feelings about them tbh, but saving water is good, to a useful point at least)

I live in Canada in the building trade and I haven’t seen one without an over flow. But now I’m curious and I’m going to look it up.