I know that modern electronics (smartphones, laptops) have dual voltage and frequencies (100-240V 50/60Hz) however when you discuss appliances (toasters, blenders, refrigerators, clothes iron, dishwashers, dryers, etc). those are region locked (besides plug type) into a single voltage and frequency. For example: if you have a electric blanket that only supports 120V 60Hz, you can’t plug that directly in a 230V 50Hz outlet as that’ll blow the fuse.
There are ways to circumvent that restriction but involves using transformers to lower the voltage (step down from 230V to 120V), as stuff like hair dryers or microwave ovens can be damaged upon using the wrong grid. If moving abroad permanently, it’s best to purchase new appliances that supports their electrical grid rather than importing them if you have only 120V versions whilst living in a country where 230V is standard.
It costs more to make things multi-voltage. For small electronics that run off DC, it turns out that the ‘costs more’ for having an auto-switching power brick is less than the ‘costs more’ of dealing with stocking multiple different power supplies to be sold to different regions.
With higher-power appliances that don’t just run off DC, where you actually need the thing itself to be different, it turns out that having multiple variants (or only selling to one region) is cheaper.
Toasters are a great example because you’re pretty much just shorting a wire into the plug, but it’s a wire whose resistance is very carefully chosen so that it gets hot but doesn’t burn up for the voltage of the grid it’s being plugged into. To make something like that multi-voltage you would need multiple paths for the power to flow and some sort of ability to switch things, all of which costs more money than not having that.
Of course, that’s all ‘in general’. It’s always going to come down to the specific factors related whatever the thing is, who’s making it and who it’s trying to be sold to.