What type of replacement bulbs are you using and are you able to describe the condition of the burnt out units?
SilverStar Ultra and some of the other higher intensity bulbs are known to have a shorter life. The Sylvania folks will even tell you, if you phone them, that they will replace only one pair during their one year warranty. After that, you are on your own.
If the bulb's glass is black or smoky, the failure caused is likely due to the bulb surface being touched, resulting in a hole/crack from the heat.
If the filament has what looks like a drop of solder on it where is is burnt out, it may be a voltage problem - too much or two little.
If the filament has what appears to be a sugar granule like coating, the bulb is old and has likely just burnt through all the hours is had available.
It should also be noted that bulbs become less effective as they age. For this reason, it is suggested to change your bulbs in pairs (both high beams or both low beams). You can then hang onto the old but still working bulb as an emergency replacement.
If you are not getting the life you expect from a bulbs, change brands. Any brand of H7 bulb will fit either the high or low beams so you have lots of choices.
I got about 30,000 miles from my OEM low beams and then only 10-15K from the Sylvania SilverStar Ultras I replaced them with. My high beams on on most of the time, running on a modulator. They are still the OEM bulbs with over 50K on them now.