Lowest Common Denominator
Found myself wondering why a telnet port would be left open in the design of this thing, and then realised that it's been built down to a price - and security costs money.
Ergo, the company that makes these has an eye on costs.
Ergo, their other products will also have the same design ethos.
And so will anyone else in the same market (if they want to compete on price that is).
Unless/until someone important (hah! define that one!!) or maybe a lot of people get severely damaged/worse by this type of design flaw then adding cost (security) into a huge range of small cheap devices will just not happen.