Why is it necessary to enter into a state of Ihram from Germany to perform Umrah in Jeddah, and why cannot one enter into a state of Ihram from Makkah Al-Mukarramah?

It is not obligatory to wear the Ihram garment from Germany. Rather, it is permissible from your country or from the Miqat. As for the intention (niyyah) for Ihram, it should be made at or slightly before the Miqat you will pass. The Prophet, peace and blessings be upon him, specified the Miqats for Ihram for each country. So, whoever comes from the direction of Medina should enter into Ihram from Dhul-Hulayfah; whoever comes from Syria, Egypt, or Morocco from Al-Juhfah; whoever comes from Yemen from Yalamlam; and whoever comes from Najd or Ta'if from Qarn Al-Manazil. If someone resides closer than the Miqat, they should enter into Ihram from their place. It is not permissible for those who are not residents of Mecca to enter into Ihram from Mecca, except for those who intend to do so from there.