Why not?
I see fewer and fewer reasons to need a computer for what most people do. So why not use the power and features of your phone or tablet instead of maintaining a computer?
Is it hard to hold the device while you type on the screen? Answer: You can just prop it up (many stands are available) and use a standard wired or wireless keyboard/mouse. Most phones and tablets allow this.
Is it hard to see because of the small screen? Answer: You can cast it, or connect it, to a big screen. Most phones and tablets can wirelessly cast pictures, sounds, and videos to a nearby smart device with a bigger screen. This works great for reading and streaming, although there is a bit of a delay which can make mousing and keyboarding difficult.
If you want to go “all the way” and use a keyboard, mouse, and monitor with your phone or tablet, you will want to plug those peripherals into your device. Most Android and Windows devices allow this.
I was recently impressed with Samsung’s DeX Pad, which allows you to plug a Galaxy S8 or S9 smartphone into keyboard, mouse, and monitor … effectively turning your smartphone into a functional desktop computer that fits in your pocket. Using this setup, the icons spread out to give you a full-screen experience, DeX-aware apps expand to use the available “real estate”, and non-aware apps still work fine in smaller portrait or landscape windows.
Of course, if you would rather separate your phone/tablet and desktop experiences, there are inexpensive options that make it easy to switch between devices: Chromebox, Chromebit, Chrometop, and Chromebook are all relatively inexpensive devices that give you a similar experience to Android/iOS without the complexity and expense of Windows or OS X.
Yes, there are situations where you just need the full-meal deal, but those situations are dwindling. More and more, traditional desktop/laptop programs are being “replaced” by web-based services, or Android/iOS apps, that are equivalent.
Even gaming and virtual reality (VR) are possible with a smartphone: the hardware cost is a fraction of computer-based gaming/VR, and the experience is nearly as diverse and impressive.
It’s something to consider the next time you go to replace your aging Windows or Mac computer.