By this standard why single out Galileo? Euclid "plagiarized" Elements, there isn't a single theorem in it that can be reliably attributed to him, and there are entire books that can be attributed to early Pythagoreans, Eudoxus or Theaetetus. In the whole 13 books he does not credit a single person by name. Descartes "plagiarized" analytic geometry, after all Oresme and Casale were drawing coordinate graphs before him. And Newton sure "plagiarized" most of Principia, the first law of motion is due to Galileo, the second one was formulated by Huygens (in the integrated form), and the inverse square law was discussed by many astronomers in the 17th century, not all of whom he credited. The same with calculus, Archimedes was finding areas and volumes, and Apollonius finding tangents in antiquity, not to mention Fermat and others in the 17th century, and even the "fundamental theorem of calculus" was already proved geometrically by Newton's teacher, Barrow.
In all cases the difference is between isolated ideas and a systematic theory. Galileo not only derived the time squared law without the mean speed theorem, but put it into a broader context of new mechanics, and applied it to physical reality, none of which Merton school, or even Oresme, did. In a 1639 letter he writes:"But in this, I may say, I have been lucky; for the motion of heavy bodies, and the properties thereof, correspond point by point to the properties demonstrated by me". Here is from Screnes's essay:
"Galileo’s calculations relied on an ancient Greek theory of proportion that was not preserved in standard medieval translations of Euclid, and didn’t become available until a better translation was made in 1543. Recent research confirms his claim that he derived the law of acceleration arithmetically, independently of experiment. His derivation was also independent of the mean speed theorem of the Merton school... In the case of the 14th century mathematicians at Oxford, a mathematical treatment of the concept of uniform acceleration was developed from which the times-squared law can be derived directly. No one in the Merton school actually derived the law, however, and there was little concern for applying the mathematics to physical systems."
Science is a multi-generational enterprise. Some of this sentiment comes from the popular but unfortunate manner of presenting its history as a sequence of discoveries and inventions that came out of nothing in a flash of insight. Every idea has precursors, and every genius has predecessors. Not all of them can be cited, or even clearly identified, so in a sense everybody is a "plagiarizer". When Newton said that he was "standing on the shoulders of giants" he unintentionally admitted it (the intention was a slight at Hooke's small stature).