Because, in order to ship something large, you have to acquire permits, commit appraisals, make sure that you comply with the quarantine standards of the target country, et cetera. There is also paperwork involved, and it is most likely too much to bear even for larger companies that need something big shipped.
You should be aware of what the company does and what it does not. If you are selling heavy-duty goods, offering international shipping would be a reasonable thing to do, and you should definitely outsource freight forwarders that will ship it.
Pick wisely though, because most freight forwarders offer additional Universal Plant's Services that can make your life easier they can calculate the cost efficiency, inform you about the customs in the target destination, supervise the loading process, and even make temporary changes to the equipment to save in shipping costs.
Another good example would be turbo equipment transport although turbo machinery is rarely sold, companies that offer rental of their mining resources know that shipping heavy equipment can become an ongoing problem if they don't pick the right service provider.
The company has to have international ties otherwise turbomachinery freight can be vastly delayed. The moving service also has to have contacts with the local companies as shipping expensive equipment can be dangerous within regions such as Russia and Latin America. You should always receive a detailed report on how the shipping of heavy equipment goes.