There are reasons that they are not readily available. And it is because they shouldn't really be used in most cases.
Having 3/4" water pressure times 2 (hot/cold) is going to blow out filters and gaskets for toilets and sinks which are not really manufactured for that much water pressure.
I had a sink at my old house - which had bad water pressure in general - and basically took out all of the prohibitors (filters/flow valve) so that the water flow was as much as possible. It was for sure too much when all the way up. And that was 1/2"x2.
You hook 3/4"x2 to a faucet and chances are you will blow the innards out over time and it will either clog (calcium plus things not in right position) or it will just allow the whole jet out. Which will be like washing your hands with a fire hose. I have walked into public/business bathrooms and you can tell within 1 second if someone ran 3/4"-1" to a sink. Water is everywhere and people are scared to turn sink on. Its not the faucet's fault because it might have help at first until its gaskets were blown.
For a toilet it is the same deal. You will be replacing the fill valve all the time if you don't regulate the 3/4" flow (even though its just cold). They simply don't make these sorts of things for that much water.
Now can you find a NASCAR toilet to take in that much water... one powerful toilet. Sure. Can you find sinks that push out water like that OK? Sure. I just bought a basin faucet made for high flow. And I got the 3/4" shutoff valve the same place I got the faucet - local plumbing supply.
So the answer is - you do need to unless you are going to install special faucets and toilets that can handle the water pressure or that you can ensure that you will regulate the water pressure using the shut-off (which is not a good idea).
Note: You should be installing 3/4" pex for hot too. I don't even count a toilet into plumbing equations so you have a sink and shower. They could consume at the same time equal amounts of hot and cold. I see no reason why you would install a smaller hot line. This could cause issues with a shower valve especially the ones with anti-scold mechanisms.
Basically if you install like this someone is using the sink. They have hot and cold going pretty hard. The person using the shower will have all the cold they want but may run low on hot. Your shower valves natural reaction will be to lower the pressure. Old school valves will just make it cold. So a bathroom should get 3/4" and branched to 1/2". You can branch to 3/4" to shower depending on the valve you get.