I cannot believe there is no demand - SIP is a system to call any phone, including mobiles, so it must be logical to be able to text that number. There are several VOIP providers who think differently to you, but no softphone that I have yet found.
Adopting your argument would mean you would be unable to call a mobile number on SIP as there is another system to do that called a mobile phone!
As for "You need to look at your VoIP account like a telephone line" - my telephone line will SMS a mobile - perhaps your's won't?