Pronouncing "mahayana"?

In the word "Mahayana" the second and third A has line above it suggesting it should be pronounced long but it's pronounced short for all the A's when most people talk, so why is it spelled this way with a line above the second and third As?

Go to the link below the site pronounces it for you.
http://www.merriam-webster.com/dictionary/mahayana

The second and third are to be splat indeed long.

'Mahaayaana'

The Sanskrit pronunciation is with the second and third A's pronounced long (and with the stress on the third), but in English, most people are unused to, or even unable to, pronouncing unstressed vowels long (or making a conscious difference between long and short syllables at all), so they usually simplify the pronunciation. Usually in English, except in academical literature, it is also written without those lines.