Because an "A" before an "R" often sounds more like an "E". Examples:
Mary, Clara, Gary, clarity, area, stare, care, clarinet, embarrass, Harris, daring, &c.
The "AR" in those words in many, many regions is pronounced just as though it were "ER", just as "AIR" is in the following examples:
Air, dairy, fair, chair, McNairy, Baird, pair, &c.
If you don't believe me, listen to a few dozen American television programs. You'll hear it a lot.