Why is there no simple function for bolding font?

"Bolding a font" is considered totally wrong in typography, so Adobe apps don't do it. You need to choose the bold variant of the font, and if you don't have one, Illustrator won't do "fake bold" like other apps (eg Word) that aren't designed for professional typography. 

Jonathan,

In addition to what Larry and Carlos and Monika said, it is always better to use a true bold version, but lacking that option it may be less complicated to fake a bold font than you believe: you can just (normal) Select the Type and then Add New Stroke in the Appearance panel; you can try out the least worst Stroke Weight; it is a bit like fake Italic (Shear in the Transform panel).

The faking is least worst with simple non serif fonts/fonts with (more or less) constant width of the letter parts.

You have to install a font that has a bold face. Illustrator doesn't make fonts fatter.

AI is very particular about the fonts it uses. To bold a font you need to click and hold on the font name in the Character panel and it should reveal the various versions installed of the font. Simply drag down to a Bold version.