Why do we read left to right?

That is the way of our tradition.
It is not the same across the globe of course.
Greeks read "as the ox plows" or back and forth across the page.
The Irish were responsible for word spacing and paragraph indentations.
Hebrew is right to left and Chinese is top to bottom.

TRADITION !!!!

Why indeed, when some writing systems read from right to left? Some even read alternately from left to right, and from right to left - saves time in shifting your eyes back to the beginning of the next line.

I think that I have read that some writing systems read down from the top, rather than side to side.

Hebrew and Arabic are read right to left. Some languages in East Asia are read from top to bottom.

Writing left to right you can see what you just wrote. Writing right to left you have to move your hand to see the previous text.

Because when we read right to left we can't make heads or tails of it.

Ti fo sliat ro sdaeh ekam t'nac ew tfel ot thgir daer ew nehw esuaceb.

Hebrew and Arabic are read from right to left.
East Asia, Chinese , etc.are read from top to bottom;at the same time from right to left/Or left to right.

Signs were erected for Roman functionaries and legionaries to read as they were returning to Rome. They always marched on the left of the roads, a custom which resulted in the British 'driving on the left'. This meant that in the Western Empire, writing was read from the left to the right, but in the Eastern Empire ( where we now find Hebrew and Arabic alphabets) - the signs would naturally be read from right to left.

just how it is

That is how our books are written.

What you mean "we," white man?

We do math equations the same way. Compose and read music too. Having a standard methodology lets everyone understand.