This is not always the case, as the Defoggers aren't necessarily superior to Rapid Spinners as a whole. Teams that do not wish to get rid of their own Stealth Rock (especially with Talonflames about) may opt for a Rapid Spinner instead, and the main choices for offensive teams would be Excadrill, Starmie, and MBlastoise.
Excadrill just happens to have a typing that synergizes very well with SR-weak mons. It also OHKOes Aegislash with LO STAB Earthquake, which is an extremely appealing trait for a spinner to have. Gourgeist does stop Excadrill cold as do burns, but the main problem Excadrill has when it comes to spinning is how it often falls prey to faster attackers due to its weaknesses and low bulk. The likes of Lucario, Greninja, Talonflame, and Garchomp, all of which can swiftly wipe it out before it can spin.
Starmie can either opt for a offensive LO Analytic spread, or a bulkier set with Recover. Essentially, the middle ground between Excadrill and Mega-Blastoise. Analytic sets pack a punch, and can nail an incoming spinblocker incredibly hard with the right move, or it can fish for Scald burns on said spinblocker and outlast it with Recover. Starmie is also much harder to stop from spinning due to its high speed, so denying the spin via revenge killing isn't always so simple. Starmie's main flaw is its Ghost and Grass weakness, meaning Analytic sets need better prediction, and the bulkier sets can't actually stay in on the spinblockers for long.
Mega-Blastoise is a tank of a spinblocker. Mega Launcher Dark Pulse sends chills down the spine of any daring spinblocker, and it is almost impossible for any spinblocker to actually beat Mega-Blastoise one-on-one, especially an offensive variant. While it theoretically boasts the most successful spin, it costs the team's Mega-slot, and being Mega means it cannot wield Leftovers: bad for any defensively minded Pokemon.
Well I've listed out the pros and cons of the offensive spinners, which would hopefully enlighten you to how these spinners are just as deserving of a spot on any team looking for a Defogger (unless they don't use hazards at all, which i say use the Defogger by all means). It's not like the Defoggers don't have their own share of flaws: Lati@s and Scizor can get trapped by Tyranitar and Magnezone respectively, and Mandibuzz is more or less a free invite for Fairies to come in. Even defensive spinblockers, such as Tentacruel and Forretress, have their merits, but I think you get the message here.