Because the computer was built to a price and there's no way they could have included a proper keyboard for that price. The ZX81 kit was £49.95 (£204 today) or £69.95 assembled (£285 today). Note that 15% of the price was tax, and if sold through retail probably half went to the shop.
(I'd love to see a full 1981 BOM for the ZX81, but finding old prices of components is tricky)
No I get that. But once you get to a certain point an ecosystem develops where you have small game Devs who would probably want a nicer computer, you have the small business owner / parent who learnt to use the computer, has a bit more to spend, has a legitimate use and is willing to spend a bit more.
Even if the price quadrupled if someones relying on that hardware, and the keyboard makes their job easier, it seems it would sell.
Plus, compared to the 90s £285 is cheap. I wish my computer's were that cheap back then. My first computer was over £500 (so what £1000 today?) and it wasn't top of the line, and I had to put it together myself.
(I'd love to see a full 1981 BOM for the ZX81, but finding old prices of components is tricky)