Why does it have to be 38 degrees out for Opening Day?

I'm going to the Red Sox home opener at Fenway today, followed by the Celtics vs the pathetic Sixers at the TD Garden.  I'd like to wear these and change shirts in between:

Or possibly my green outfit that works for both teams:

Unfortunately I don't think the cold weather will allow for either.  Why does MLB do this?  Less than half of its teams have outdoor stadiums in places that are still cold in early April.  Why not just always play the first few weeks of the season indoors and in warm weather?  I just don't get it.

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