In elementary school, it's hard for kids to "switch between notation", as we have done later on in our lives.
We start with $\div$, then we go to $/$, and now we do something like $\frac{a}{b}$.
In elementary, students are not familiar with "fractions", or just know the very basics about them. Hence the $\frac{a}{b}$ ratio doesn't make sense ... not until you actually use fractions in middle school and so on.
It's important to realise that $'/'$ the slash, refers to fraction. When we write $(a/b)$, it's because we are lazy and do not want to write $\frac{a}{b}$. So technically it's the same thing as fraction.
This is why students in elementary just use the regular $\div$ symbol. It's just a symbol. No fractions or anything complicated just yet. It's the same reason why kids in elementary use $\times$ to indicate multiplication and not the dot, $\cdot$, as we use when we are older. They do not deal with complex numbers, and so these symbols that we use as adults are really after we see the use of math. In elementary, it's very basic, hence basic symbols.
In the end it's because of the type of math we do. Imagine writing something like this:
$$\int^{\dfrac{x}{5}}_{0}(x^3+\frac{x^2}{5}+\ln(\sin(\frac{x}{4})))dx$$
$$\int^{x\div 5}_{0}(x^3+(x^2 \div 5)+\ln(\sin(x\div 4)))dx$$
We even had to add extra parentheses to make it clear that $x^2\div 5$ is one thing. Heavier math = more compact symbols
Best Answer
$\angle$ is usually used to denote a standard angle, whereas $\measuredangle$ is used to denote a directed angle.
That is, given two non-parallel lines $\ell$ and $m$, the directed angle $\measuredangle(\ell, m)$ denotes the measure of the angle starting from $\ell$ and ending at $m$, measured counterclockwise.