The leftist bloc may have agreed on Lucie Castets as their candidate for prime minister, but Macron has a constitutional duty to ensure the stability of the country. If he were to appoint Castets, all 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.
That Macron and his centrist allies are still in denial doesn't affect the fact that the French people have given a clear mandate to the leftist NFP. Therefore, his moves to deny the largest alliance its chance to form a new government and grab power through covert plots indicate his undemocratic disregard for the popular vote.
Macron is on a dangerous path against both the most voted party — the right-wing National Rally — and the largest group — the NFP — in a desperate power grab after his camp suffered a massive defeat in legislative elections. As he does so, he denies democracy and undermines the republic's institutions and the sovereignty of the people.