Because these licenses require something other :
The MIT license is a ‘template’ license: it must be instantiated by the intended copyright information. The license then requires that both — the copyright line and the license text — is distributed together with the open source program: “The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.”
The GPL allows to distribute the source code “provided that one conspicuously and appropriately publishes on each copy an appropriate copyright notice and disclaimer of warranty […] and gives any other recipients of the Program a copy of this License along with the Program”. Additionally, the GPL requires that each code which uses the GPL licensed library etc. is also licensed under the GPL (copyleft effect)
How can we resolve this inconsistency?
It is clear, that there exists a contradiction between the claim of the licenses and the everyday practice:
On the other side, usually the compressed library does no longer contain the required license information: the smaller the lib, the faster the download.
But it is useful, that nearly each open source license requires the distribution of itself — together with the copyright information. That’s not a legal gimmick! Only who has received the license can be sure, that he really has the rights the copyright owner want to grant — if these copyright owner are known and named in the licensing statement. Therefore, it is meaningful to distribute also the copyright line.
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