The real reason for not allowing
\theta in normal text has to do with how TeX deals with fonts.
A TeX font has only 256 slots, while the mathematical symbols are many more. The command
\theta is, essentially, a four digit hexadecimal number:
which carries a good deal of information: the leftmost digit is 0 and tells TeX that it's an ordinary symbol; the next digit tells TeX what font it should draw the character from and the final two tell where in the font the symbol lives (place
0x12=18). Up to sixteen fonts can be used for math symbols and they are not necessarily tied to the text fonts (actually font number 0 should be a text font, but it's an exception).
TeX might have been designed so that a single command denoting a math symbol did the choice automatically, but the effects would be adverse in most cases.
One should also remember that math has its own spacing rules that TeX applies automatically. Letting people write math symbols in normal text would soon lead to poor typesetting.