Everything has been worked out in the metric system too, so it should not matter.
By the way, when you refer to the cup, teaspoon, and tablespoon, which ones do you have in mind? The units vary in size from one country to another.
Fractions are simply easier to work with than decimals when it comes to cooking.
I doubt it matters much either way. Most volume measurements in cooking are approximate, and complicated calculations are rarely needed. You can talk about a quarter liter if you prefer fractions, or 250 mL if you don't; it is the same volume.
Why do you think eggs come in a dozen? It is easy to get the fractions out of a dozen eggs. It is hard to get the fractions out of 10 eggs. I can get 1/3, 3/4, etc. out of a dozen eggs but I can't get them out of ten (all I can get is half or tenths).
Yes, as I previously wrote, dozens are good for counting, because 12 is evenly divisible by 2, 3, 4, and 6. Objects are counted the same way whether you use SI, Imperial, or U.S. customary units.
But counting is not the same as measuring.
The metric system simply isn't efficient when it comes to cooking.
I suppose that depends on who is doing the cooking. Someone who is used to the metric system would undoubtedly find the Imperial or U. S. customary systems to be inefficient.
Though for many bakers, they use weight instead of volume when baking (so they could easily use a metric scale).
Actually what you will find, is that in cooking, the metric equivalents of the cup and the teaspoon etc. have just been converted over, because those quantities are naturally occurring. There is no advantage with the metric system for cooking and there are some disadvantages.
The same argument occurs with measurements. A nautical mile is much better than a kilometer for navigation purposes.
The only place where the metric system is superior is in scientific and engineering calculations. Just shifting the decimal is clearly better than imprecise conversions.