because we are human and we cannot play perfect and noone knows that perfect chess is a draw since chess isn't solved 100 %.
humans make inaccuries and players like Magnus Carlsen can grind a win in positions that appears to be drawish. Too many GM would just a agree to a draw in those positions.
It is pretty well known that with neither side making an error-chess is a draw.
With supergrandmaster making more and more draws and with correspondence chess with the help of computers it is very apparent that the end result of a well played chess is a draw.
Some correspondence masters know they can draw as Black after this sequence: 1. e4 e5 2. Nf3 and from here Black can hold a draw in Centaur Chess.
My answer is 99% of all players play for the enjoyment they get out of chess even though some know the game should end in a draw.
In other words there may be drawing lines or lines which turn out to be very equal in many openings but very few players know such lines and some such lines are very complicated.
So, for most it is the creativity and being able to plan ahead and the thrill of finding a win [or a draw] and using your brain is great stuff.
Even if an opening is said to be objectively bad it can be great fun for aficionados who like to play the opening. An example of this is the Blackmar Diemer Gambit which is ranked about 10th of of more than 40 some openings as giving good practical chances. It does not matter so much that there have been given lines against that opening which seem to favor Black--you will rarely run into such lines and in the meantime have great fun.