According to "What do adult squash players think about protective eyewear?" (by C. Finch, P. Vear):
The major reason for not wearing protective eyewear was the perception
that it was unnecessary. Poor vision and a lack of comfort were also
stated as reasons by a significant number of players.
BBC article "Squash players fail to protect eyes" states as:
[...] among those not wearing eye protection, over a third complained
that it restricted their vision during play, and one in four said it
was too uncomfortable, even though most had not actually tried it.
Pro-players are being injured too. You may read SquashClub.org FAQ on "Are eyeguards neccessary?":
Can you cite top players who have been injured?
Jonathan Power was in top form and looked like winning his second
World Open title in 2002. Up a game in the semifinal with David
Palmer, he was hit in the eye with Palmer's racquet. Power did not
suffer permanent damage, but he did not open his left eye for some
weeks, and had to retire from the match.
Alex Gough was up a game and beating John White in the 2006 Motor City
Open Semifinal when White hit Gough in the eye with his racquet. "Play
was suspended for five minutes as Gough was treated...Gough's eye
continued to ooze blood for the remainder of the match...The blow,
coming late in the second game...cost him the point, and the momentum
- as White prodeeded to dominate the next two games to win."
Julian Wellings hit Nathan Dugan square in the eye with the ball
during a freak fit of anger, and Dugan's full vision did not return
for three months.
Will Carlin, former U.S. champion ranked as high as #60 in the world,
received a detached and torn retina from a ball strike. He has since
endured two long surgeries, $50k+ in medical expenses, extreme pain,
nausea, anxiety and a flood of "floaters." He also lost two years from
a promising professional squash career. Please read US National
Champion, Will Carlin's story.
Please also note that, according to the US Squash:
The risk of eye injury in racquet sports such as squash is high
according to the American Academies of Ophthalmology and Pediatrics,
the American Optometric Association, and eye care professionals who
have studied sports eye injuries. Fortunately, these injuries are
almost totally preventable with appropriate protective equipment.