Why do we call $\sqrt{-1}$ imaginary and $-1$ real?

In mathematics, every modifier, such as "continuous, real, rational, ...", is reassigned meaning, lest there be unnecessary confusion such as "your 'continuous" is different than mine" that wastes time. So, if you come across a terminology in some math book, find its definition within the book; do not take a dictionary and check the meaning of the term...

I am not sure if you are asking about the origin of the reason why the word "imaginary" was chosen. If not, then leave the "secular" meaning of the word alone. If you do this, then it is crystal clear that "imaginary number" is no more than a word used to name the relatives of $\sqrt{-1}$. Note also that "positive" and "negative" in math have nothing to do with any sentiments.

Logically, you can call the real numbers "imaginary" and the imaginary numbers "real". Then in your book, the theorem "the square of every real number is $\geq 0$" becomes "the square of every imaginary number is $\geq 0$". Noticed? The name thing is rather superficial :), which in math serves as a mnemonic device and a shortcut of reference.

Natural numbers is a very intuitive mathematical construction, corresponding to counting, and this is why they are termed natural. Together with natural numbers, addition appears.

Then the need appears to solve problems like $3+x=7$, how much is $x$ ? This is solved by means of subtraction. But then you face frustrating problems such as $7+x=3$, how much is $x$ ? To cope with these, negative numbers are introduced, forming the integers. (A close friend of entire.)

The next step is to look at multiplication, i.e. repeated addition. All is fine until you want to solve problems like $4\times x=24$, how much is $x$ ?, then $7\times x=17$, how much is $x$ ? This is how division and the rational numbers are introduced. (From ratio.)

The ancient Greeks once discovered that rational numbers are not all, much to their resentment, when they asked the question $x\times x=2$, how much is $x$ ? Then came the real numbers. (Possibly evoking the continuous characteristic of our real world.)

Another step was reached in the Middle-Age when mathematicians started to deal with the equation $x\times x=-1$, also annoyingly unsolvable, and the imaginary and complex numbers were introduced.

You may attach a "romantic" meaning to these terms, but this is not the intent. On the opposite, they are conventional and universally adopted words with a well-defined understanding, and probably a more mnemonic intent.

Even though some of these number categories may look somewhat artificial, there are cases where they come handy just for intermediate results in solving a real-world problem. A famous example is the need for complex numbers to find the three real roots of some cubic equations.