I mean if the super magical substance which the coaster is made out of is so repellent and immune to the liquid that rolls off your glass, then why dont they make the entire table out of that substance?
Cork, generally. Or glass, sometimes other types of timber.
Because cork looks crap when it’s made into a table, it’s also very soft. Some table are made of glass and many tables are made of timber. Of course there’s nothing special about coasters except that you can put them into the sink and wash them.
Most people do not mind throwing away a $10 set of coasters if they become stained but replacing a $100+ coffee table is more expensive. You don’t have to look at stained coasters unless you are using them.