Because we haven't sent several million people. We've deployed 4,000 infantry personnel to Iraq for security duties and special operations, about 3,000 to Kuwait for security duties and special operations, 7,000 contractors to both countries together, and about 500 personnel to retrain the Iraqi Army. In order to prevent the deaths of a large number of ground troops like the last time, we're focusing on conducting airstrikes, training the Iraqi Army, providing security services, and participating in an occasional special forces raid. We're letting the Iraqis and Kurds do the groundwork. Another major ground war would not be popular here at home, especially if it were followed by another lengthy occupation.
In addition to the American ground forces, we've deployed dozens of naval vessels and hundreds of aircraft. These aircraft have since flown 44,000 sorties, a huge amount of activity by any measure. 11,000 airstrikes have been launched against ISIS, with 4 times as many being launched by American forces than by all the other Coalition partners put together.
Furthermore, President Obama did NOT take office 8 years ago. He took office 7 years ago, on Inauguration Day of 2009. Operation Inherent Resolve did not begin until June 5th 2014, so the war has been going on for less than 2 years. In that time, the military intervention against ISIS has reversed their advance, cut off their recruiting, destroyed 90% of their oil production (with Operation Tidal Wave II), killed enormous numbers of enemy combatants, retaken vast amounts of territory, and saved more civilians than it has killed.
In October 2014, US Central Command published a press release stating, "Tthe name INHERENT RESOLVE is intended to reflect the unwavering resolve and deep commitment of the U.S. and partner nations in the region and around the globe to eliminate the terrorist group ISIL and the threat they pose to Iraq, the region and the wider international community. It also symbolizes the willingness and dedication of coalition members to work closely with our friends in the region and apply all available dimensions of national power necessary - diplomatic, informational, military, economic - to degrade and ultimately destroy ISIL." Other countries already see that we want ISIS destroyed. So now I dare you to find a problem with our military strategy.