Apple at least is very specific about iOS support for phones. Updates will be provided for two years after the device is last sold. Since the older models continue to be sold for a year or two after the newer models are introduced, in general iPhones are supported with upgrades for four to five years after the model is first introduced. From Apples perspective, the iPhone is an evolving platform so is continuously supported.
Samsung, LG, and the other manufacturers generally build their phones on a one-off process. A team is brought together to build a phone device for one or more carriers. The hardware is designed, and the software ported to the hardware, normally with extensive customization for each carrier. (In fact it is possible that the same "model" phone sold for two different carriers could also be completely different hardware internally as well as being heavily customized software.) A few hundred phones are built for final verification and testing by carriers and regulatory agencies, then, once approved, the production lines are turned on to build a few hundred thousand phones. Once production is complete, the factories are turned to other hardware, and the design teams, both hardware and software a broken up and assigned to the next projects. Three months later, there is no one left to update the software with security patches or update to the next version of Android. It is also likely that the additional software components, games, utilities, media codecs, and the like were licensed only for the initial production run, and are not available to be used in a subsequent update. Even if this was not a problem for the manufacturer, every update would also have to go through a full carrier review process, and carriers would have to be willing to accept the risks of damaged phones because people will try to update on 5% battery.
I have not even touched on the accounting aspect of this - accounting rules may also make it difficult to release a free update, especially if it provides new functionality, if the revenue from the sale is recognized at sale. (Unlike others, Apple changed its accounting practices to allow iPhone sales revenue to be recognized over the expected lifetime of the phone, which means it can offer free updates and upgrades. Steve Jobs ordered this accounting change when he had to charge for an upgrade he wanted to supply for free.)
Sorry, updates of Android phones are not going to happen the way iPhones are updated.