As I see it, megacorps are sometimes conglomerates with multiple fingers in many things and sometimes (somewhat) specialized in specific industries. In ye olde days we had the East India Company that controlled trade and later politics in India, or Hudson Bay company in Canada. In the US you had the various Trust companies, or the grand-daddy vertically and horizontally integrated Standard Oil. We've often heard of coal companies having "company towns" where they were the only real employer and retailer (and land owner/slum lord/etc). Shadowrun has a similar idea (and also, in a very Traveller-esque manner these companies sometimes engage in armed combat with one another).
In more modern industries you had companies like GE or GM, that have ebbed and flowed as conglomerates across many industries. The books describe Imperium-wide companies like this, and then sector and sub-sector wide (the Tukera lines and Al Morai). The Imperial-sized ones are so big that they really don't do much at the player level. Smaller-sized ones still are quite big from a PC perspective. Since Traveller is mostly an RPG game, these kinds of companies are really come off as just set-dressing. It's hard to RPG with a megacorp setting.
I think when (some at least) of the game designers sat down they went more for cool factor, or "this sounds cool" and worried less about whether or not these companies would work - and how much they may match historical models and/or realistic ones. For some players the cool factor is more than enough and they'll go from there. Others find it requires too much hand wavium.
Personally I like to find a middle-ground that matches something that makes logical sense, is interesting background, and isn't so detailed that it requires me to use a spreadsheet on an hourly basis. And discussions with like-minded people on boards like this help are good sources of different viewpoints.
Plus nobody has devolved into neener-neener mode... which makes for a pleasant conversation.