Most European restaurants don’t offer tap water because it isn't always safe for consumption. Hence, having paid for bottled or sparkling water, they pass the cost to the diners.
The most touristy countries, like France, Germany, England, Spain, and Italy, have safe drinking water. Other countries, like Romania, Cyprus, Belarus, and Moldova, don’t.
It’s often because these countries lack proper regulations or funding to ensure their water is safe. The water can also contain bacteria tourists may not be immune to.
Sometimes environmental catastrophes have affected the water supply, such as in Belarus and Ukraine, where the water supply was affected by the Chernobyl nuclear meltdown in 1986.
For the same reasons, in most European eateries, free refills are not the norm as they are in American ones. However, don't hesitate to ask for water as long as you do so politely.