The leftist bloc may have agreed on Castets as its candidate for prime minister, but Macron has a constitutional duty to ensure the stability of the country. If he were to appoint Castets, all the other groups would form a majority to vote a motion of no-confidence against her government — and that's definitely not what institutional stability looks like.
Macron and his centrist allies are still in denial, but that doesn't change the fact that the French people have given a clear mandate to the NFP. Therefore, Macron's moves to deny the largest alliance its chance to form a new government and instead retain power through covert plots indicate his undemocratic disregard for the popular vote.
Macron is playing the most-voted party, National Rally, and the largest group, the NFP, for fools in a desperate power grab after his camp suffered a massive defeat in legislative elections. As he does this, he also denies democracy and undermines the republic's institutions and the sovereignty of the people.