Your work permit is the status document allowing to work in Canada and reside in Canada for that purpose. The visa is an entry document allowing to enter Canada.
In many countries, a work or residence permit can act as a visa and no separate entry permission is required. Canada is not one of those countries. If you need to leave and re-enter Canada with a valid work permit but not a valid visa, you need to apply for a visitor visa before you leave Canada.
The only exception to this is if you only visit the US or St Pierre et Miquelon and come back. Then an expired visitor visa can be accepted in that case.
Additionally, any implied status will be lost if you leave Canada during the time between the expiry of your old work permit and the issuance of the new one. In that case, even if you have a visa, you may be let in as a visitor but you will not be allowed to resume work until the new work permit is issued.
I cannot say for sure as to the reasons behind why they don't allow work permit to be used in lieu of visa for entry from other countries (except US or St Pierre et Miquelon) for visa-required nationals.
It could be a cost-saving measure. It is also an additional check for visa nationals because the date on the study or work permit is not necessarily the actual. Your work or study permit will be voided if specific conditions are not met. E.g. If you change the employer without authorization, the permit expires. A study permit will expire 90 days after the completion of studies even if the date on the permit is longer.
Why is a visitor visa issued automatically the first time?
Because you cannot come to Canada and use your work permit if you do not have a visitor visa. It would be meaningless if they do not give you one the first time. The government also do not to verify if the work permit conditions are met if it's the first entry.
If you extend it in Canada, you do not need one unless you leave Canada.