While the other answers are correct about why GPS (or similar GNSS, such as GLONASS, etc.) aren't used exclusively for aircraft navigation, I would say that, these days, the premise of this question is no longer correct and GPS is the primary means of navigation for aircraft. It just is not the exclusive one and it most likely never will be because the aviation industry very understandably likes having backups to their backups to their backups, ideally ones that do not have common modes of failure, as would be the case with using other GNSSs.
That said, due to the extreme cost of getting new hardware certified and the slow nature of regulatory changes, the aviation industry has a lot of momentum and major changes take a long time, both to be approved for initial use and then to actually become ubiquitous. Airplanes, once built, tend to have very long service lives, especially when talking about an entire type of airplane rather than just one particular airplane. As an example, I'm in my late 30s and every airplane I've flown was built before I was born and was based on a type designed in the 1950s and/or 1960s. While airliners don't usually stay in service that long, we're still talking about production runs that last for decades and then service lives of decades beyond that.
Of the 4 aircraft that you mentioned, the second-newest of those types is the 747-8. Aside from being based on an airframe originally designed in the 1960s and designed to be flown by pilots who had been trained on previous versions of the 747, even the 747-8 itself began design in the early 2000s. The A310 was designed before GPS was even allowed to be used for civilian purposes and the A320 family (of which the A319 is a part) was also already well into its design phase before GPS was opened up to civilian aviation, though it had become available before it first flew. At the time the A310 and A320 were designed, airliners were still sometimes navigated at night in part by looking at stars with a sextant!
As for inertial reference systems specifically and why they are used in addition to GPS, they are actually great and can give much faster and more precise measurements than GPS in the short term. Orders of magnitude more precise. Additionally, they provide orientation information that GPS does not (i.e. GPS will not tell you if you are rolled to 35 degrees right bank angle and pitched 15 degrees nose-up, whereas the IRS will.) Over the course of a whole flight, though, their drift can become significant without something like GPS or radionavigation to give them occasional fixes. This is actually how your smart phone (or other portable electronic device) works, too. It uses GPS (and cell towers and other such things) for occasional fixes to factor out drift, but it uses an IRS with accelerometers and such that measure acceleration in different axes in order to work out short-term position and orientation changes. The same is true for virtual/augmented reality systems and such, except that they use optical systems for their fixes rather than GPS or radio signals (which wouldn't be nearly precise enough for their purposes.)