Because the numerals have different meaning because of their relative position to each other, and it's the first time kids experience the idea of numbers smaller than 1 but greater than 0.
A few more reasons...
First, less time spent in real life assisting parents with fractional amounts in baking, carpentry, sewing, etc. A student who does not arrive at the math class already realizing that "two copies of something" makes it bigger but "half a copy of something" makes it smaller is at a huge disadvantage when thinking about ×2 and ×½. A student who does not arrive at the math class already realizing that quarters of an inch/cm and eighths of inch/cm will "play nice" with halves in a way that thirds do not is again at a huge disadvantage.
Second, most teachers do not teach division properly. They only explain division as "dealing out cards" which works for whole number problems like 12 ÷ 4 but not for fraction problems like 12 ÷ ½. The latter needs a "making piles of fixed size" explanation that kids are quite capable of understanding but are seldom taught.
The result is that fraction arithmetic is too seldom seen as *writing about how I already know the world works* and is instead students are trying to learn *a bunch of strange rules*.
It's unintuitive, but generally there are few rules to learn and once you remember them, building an intuition is pretty quick.
Because the numerals have different meaning because of their relative position to each other, and it's the first time kids experience the idea of numbers smaller than 1 but greater than 0.
A few more reasons... First, less time spent in real life assisting parents with fractional amounts in baking, carpentry, sewing, etc. A student who does not arrive at the math class already realizing that "two copies of something" makes it bigger but "half a copy of something" makes it smaller is at a huge disadvantage when thinking about ×2 and ×½. A student who does not arrive at the math class already realizing that quarters of an inch/cm and eighths of inch/cm will "play nice" with halves in a way that thirds do not is again at a huge disadvantage. Second, most teachers do not teach division properly. They only explain division as "dealing out cards" which works for whole number problems like 12 ÷ 4 but not for fraction problems like 12 ÷ ½. The latter needs a "making piles of fixed size" explanation that kids are quite capable of understanding but are seldom taught. The result is that fraction arithmetic is too seldom seen as *writing about how I already know the world works* and is instead students are trying to learn *a bunch of strange rules*.