It's isn't. The idea that it's wasting money, or that you're paying someone else's mortgage, or whatever other argument is thrown out there are part of one of the biggest financial misconceptions going.
Do the cost/benefit of renting vs ownership, accounting for maintenance, taxes, interest rates, macro-economic conditions and whatever else and the costs should be very similar. Of course, it ebbs and flows with the markets, but at the margins is where the decision should be made. Owning is never a clear winner in all circumstances.
That said, with the drastic drop in house prices since 2007 and historically low interest rates, buying has been (in the US at least) a financial gift from the heavens for the past few years. Unfortunately, that ship has sailed.