The prevailing design requirements for certification are "old", by today's standards, but based upon simple wisdom. A lot of this wisdom has been born of experience, a lot of it from WW2 airplanes. Within these requirements, an engine for a certified aircraft, must itself be certified. That process is very expensive and burdensome. And, referring to the [present] engine certification design requirements, certain simplicities are required. One of those is that the engine must be able to run without an external source of electricity - hence magnetos. Nearly all legacy horizontally opposed piston engines are direct drive, which is simple and reliable, but contrary to modern engine design. Certainly some modern engine installations are being certified with variations on these requirements (by "special conditions"), but it's expensive. So, for a brand new airplane design, with a modern engine, it may be worth the investment. But, for an airplane designed in the 1960's, and built '60's to '80's, it may not be worth a million dollar program to certify a more modern engine, when you can just keep rebuilding the original one.
Present light airplane engines (Continental and Lycoming) are surprisingly simple, and lightweight for the power that they produce. They are comparatively reliable, when I consider the "computer" problems I have experienced with modern car engines! Many of the failures you may read about are a result of improper operation, rather than the engine itself failing mechanically. But yes, they are old and very basic in design. To get modern engines into the general aviation fleet, will require lots of money, and regulatory advancement (= money + time). The general aviation industry is not known for having excess money!