Where in canon did you read that?? Library Data A-M: "Tukera Lines operates a vast fleet of passenger and freight vessels throughout the Imperium, following the xboat lines. In some subsectors (particularly the older, more established regions of the Imperium) Tukera Lines has a virtual monopoly on long distance shipping and travel." How does one manage a monopoly on shipping without shipping other people's goods?
I don't know to what extent it can be considered canon, and it's MT, not CT, but IIRC (I have not my book handy now) in Starship Operator Manual there was quoted that those megacorporation are not freight companies, but trade companies that buy and sell goods, instead of only freighting them.
Of course I guess they also handle freight, but, as I understand them, I agree with Mike that most of its profit comes from speculation, not from freight/passenger transport (but I agree that's my guess and understanting, not canon)
Fine, now support your assertion in a manner that speaks to the examples given - namely, how a jump-4 transport comes into existence and becomes "typical of its [Tukera's] fleet" (Traveller Adventure) when it takes up to a century for the thing to earn back the money spent to build it while other alternatives could return the investment in 20 years. Clever debating phrases like "pat 'logic'" do not take the place of evidence, and saying things are "interconnected" and "humans are't universally logical" are handwavium, not explanations. Note my discussion of possible sociological explanations below.
Not to this extreme; most investors expect to at least make the money back in their lifetimes, not their grandchildrens'. Some investments don't pan out, some experiments fail, but something which is described as "typical of its fleet" is supposed to be successful.
This goes beyond design. It is impossible to make a jump-4 merchantman that returns a reasonable profit in a reasonable time - reasonable being interpreted as anything that might persuade a potential investor to put his money there instead of in any one of a billion other potential ventures. Not a case of poor design - it's a case of the rules making it impossible. Whether it's big returns for big risk or small returns for small risk, someone investing for profit does tend to expect a return in their lifetime.
I have entertained the notion of significant cultural differences: that these people make their billions in some other way and then enforce fiat pricing rules and sink money into things like jump-4 merchantment out of some sense of noblesse oblige or "rich man's burden" rather than in pursuit of profit, in an effort to actively promote an interstellar economy and society. In short, they are more social engineers than yankee traders. Perhaps history has taught them that actively promoting interstellar trade results in fewer wars and fewer bombs being dropped from orbit, an unhealthy state of affairs for any billionaire's investments.
I have not run the numbers, but I guess those same jump 3/4 ships would do some profit if paid in full, without financing (financing the ship acounts for a 120% ship cost increase). If so, I guess a megacorp like Tukera could well buy its ships without financing them, investin part of its profits on it, and so the main cost will be forfeited (or, if you want to be precise, reduced to a 41.66% of what rules say, and so paying the initial investment in the usual 40 years) allowing it, along with some speculation, to make some profit.