Or perhaps that type of cover is either unavailable or
extremely expensive...
After all, Insurance companies are in the business of making money... they will not insure against things that are - shall we say -
likely to happen.. unless of course the premiums are so high as to make their exposure negligible... (Car insurance rates for a 17 year old male on say.. a Lamborghini

)
The probability of misconnecting when one has opted to book "connecting" tickets on two different airlines (not partners).. and the subsequent expense of rebooking "use or lose" fares MUST be fairly considerable.... so IMO it isn't hard to see why insurance companies may not want to take on such risks....
After all, we are clearly discussing the conscious decision to book the riskier option (the one where no airline is responsible for our onward travel).... either
to save money (
LCC was mentioned) presumably or to "suit ourselves" for whatever other reason......
A story (probably apocryphal... but never let the truth stand in the way as they say) from the Canberra bushfires some years ago tells of a resident of the suburb of Duffy calling an Insurance company to urgently insure his house...... The insurance company wisely declined - as many houses in that area had already been shown on TV going up in flames...
He was (as the story goes) irate, and demanded to know why they wouldn't accept his premiums....
Travel insurance is difficult , and expenisve to obtain.. for those over 72 years of age... it isn't hard to understand WHY...
This seems to be a similar situation to me... the risks of a claim being made are high enough to make it unprofitable/unworkable for the insurers...