Good point. I agree.
Old laptops with real RS232 serial ports and parallel printer ports can be interfaced to a variety of useful external hardware with cheap discrete components like resistors, diodes, transistors, and capacitors. But, if you have more modern laptop that relies on USB, the lack of GPIO pins is an obstacle.
Another problem is that old laptops often have the hard drives removed (destroyed for security purposes), and the batteries are too old to hold a proper charge. The battery problem is important because it can prevent the real time clock module from maintaining the correct time and date. Knowing the time and date is essential for controlling irrigation and lighting. Refurbishing old computers to the point of being useable is often not cost effective. I’ve spent a lot of time on eBay browsing for things I might buy as a starting point for projects of this nature. But, the math usually doesn’t work out. By the time I acquired all the parts I would need for a working system, I would be better off using new stuff with better specifications. If you had access to boatloads of free surplus equipment or e-waste, it might be a different story.
For phones, people have done interesting work to interface sensors using audio signals into the microphone jacks. But, bluetooth is much more common as a way of connecting phone peripherals. Building bluetooth peripherals to work with old phones would probably be a lot more hassle and expense compared to just building a device based on Arduino, Raspberry Pi, or whatever, that did the irrigation or light control tasks directly.
If I were to attempt a project like this, I would probably ignore the phones and look for old laptops or PCs that had the old style serial and parallel ports and a good reputation for being easy to service. I would plan on needing to replace the clock batteries and add new hard drives. For PCs, I would plan on needing to add battery backup power supplies (UPS), and for laptops I would plan on adding new power adapters and battery packs. That said, I would be a lot more inclined to just use a microcontroller with a real-time clock module because it would use a lot less power, be more reliable, and generally be less of a hassle. That’s basically why Arduino has become so incredibly popular.