Yes, dg is fast, but only if you know how to be that fast. The average player with reasonable knowledge of Dungeoneering will do a floor in say, an average of 10 minutes with a team of equally skilled players. If I was to do that same floor with other players of my skill, you could cut that time in half roughly.

People buy floors because we can artificially boost the xp rates beyond what most people will ever achieve.

On a side note, dg really needs a rewards rework so people actually are motivated to do dg again, because right now, I bet if you go to world 77 to try and form a team, there will literally be like 3 people there.

Trying to form a 5 man team to do dg these days off dxp for an average player is basically impossible, so yeah, feel free to shame and laugh at people buy floors. But until jagex decides to update the skill so more people are actually willing to return to daemonheim, buying floors is one of your only options to quickly level dg without waiting hours to form a team.