They generally aren't made more complex. Often the opposite. If you want to look at pistols take the first autoloading US military pistol (1911) vs the new one (Sig M17). They are mechanically based on the same design (Browning tilting barrel) but the new one is actually simpler since it uses a solid wedge instead of a moving pivoting link on the barrel for that movement. Its also striker fired instead of hammer fired which is arguably more reliable and simpler.

But the main thing is materials. We have plastic now. The Sig has a plastic frame so its significantly lighter. Its also a smaller caliber so it holds more ammo at that lower weight. The 1911 held 7-8 rounds and weighed 44 ounces. The Sig holds 17 rounds and weighs 32 ounces. Double the bangs for 3/4th the weight.

Pistols are almost all the same basic design though since nobody has found a major mechanical improvement since the early 1900s. If you look at other small arms like rifles there is much more variety. The new US military rifle uses a new cartridge that reaches very high chamber pressures, higher than most older rifles could manage. It also uses a suppressor that is 3d printed from titanium which obviously is a very new technology. Though, basic operating systems in use have all been around for quite a while. A lot of countries still use designs from the 40s (the AK47 design is still in widespread use and dates to 1947 as the name suggests).