It's strange to me that this is how TeX creates \Longrightarrow. It seems inelegant by TeX standards.
You have to remember that back in the late 1970's, when TeX was developed, memory and disk storage was much tighter, and so TeX had to use the smallest number of characters in its fonts that it could. Using multiple characters like this allowed more glyphs to be produced without making the fonts larger. It also makes it possible to have "stretchy" arrows, where the equal sign is repeated multiple times to make the arrow long enough.
Most of the arrows were produced this way (hook arrows had a separate "hook" character, mapsto arrows have a separate tail, and so on).