When I'm learning other languages, I usually (99% of the time) find that "England" is either the same or very similar in the other language.
This is incorrect. We do not distinguish Britain, UK and England, at least not in the common sense of Inselaffen, informally speaking. In political terms we may be more particular: Not every Englishman is Angelsachse, but nobody is counting on the difference.
Many political entities and their names are newer than that, and more foreign1, which explains why there is less variation. England or China for example are just not very good examples.
1: An often repeated Heuristic after Bartoli says, the center innovates, the periphery remains conservative, cf. Trudgill on "Sociolinguistic typology and the speed of linguistic change" in Journal of Historical Sociolinguistics, vol. 6, no. 2, (2020).
The same principle holds for Germany and German speaking regions. The political divide is always fairly subjective and technical or emotional2.
2: “Bayern gehört zu Österreich” is banter heard in the north of Germany. Mittelbairisch is indeed spoken in both countries. “Piefke” on the other hand is an offensive or meliorative Austrian German word for Germans, potentially including those from Bavaria on political grounds. Both agree that northern Germans are “Saupreußen”. “Saksa” is reportedly used as a slur in Finnland. Ethnic slurs are not scarce at all and often reciprocal. The modern use of “Nazi” seems to continue the older sense of Ignatz > Nazi ("idiot"). The political tangent from National Socialism is barely metaphoric as in “grammar nazi”.
The question is just too broad:
Is there a reason why Germany (Deutschland) is called so many different things in other European languages?
There are many reasons. Not all of them are actually reasonable
It may be argued that “Deutschland” is a metonym. For example, “Germany has voted” ipso facto defines Germany as the sum of subjects who have voted. I disagree because it would mean that I either am not German (false Scotsman fallacy) or I voted for something I haven't actually voted for (another fallacy, not understanding metonymy). Yet it is entirely lexical to say so.
Other languages have simply calqued from usage. They haven't borrowed "Deutsch" because the name is fairly new in reference to geography and it's too easy to confuse with the name of Dutch, from the same word. The word may be older but it had different meanings, which is a subject for another day under another question. It had therefore many different possible calques depending on the meaning in each instance. Besides, it is used today to calque different words.
Other Germanica are off-topic3, except when they may be explained in German. So for example among Family names, Niemczyk, Nimschik etc. are well attested variants to Niemtschke, Nimtz, Nimz, Niemetz etc. (Polish Niemeć "German”) beside Nie- for neu "new" (Niemeyer; Dutch Nijmegen ~ Ger. "Nimwegen") or Nier ("nieder"), Neymar (?) and what not; Meijering is but a form of Meyer, from maioris, mega-, compare michel (Middle Low German "big") rather than Mikael, Mike, Mitchel; only Neumerkel comes close to Mark, which has several explanations (Digitales Familiennamenwörterbuch Deutschlands). This would be my best guess at relating Niemeć to Morvec, Morvek, Moravia. That's impossible without some heavy lifting which the Erbfeinde on all sides will fight tooth and nail. Further, Frankish Merowingian is impossible to pin point.
The origin of the Germanic tribes is lost to history (thus Wilhelm Schmidt, Geschichte der Deutschen Sprache, 12. Auflage. (2020). p. 45).
That said, the Polish suffixes may be derived from Proto-Slavic, from Proto-Balto-Slavic (PBS)
-iec /jɛts/ << PBS *-ikas,
-(i)eć /ɛt͡ɕ/ << PBS *-ḗˀtei and
-(iec)ki << PBS *-iškas; niemiecki "German".
(en.Wiktionary)
Although, the palatalized consonant ć before high vowel may be identified with palatalized Swedish tjod [ˈtɕʌð], Old Saxon thīod, Gothic þiuda < PG *þeudō, Swedish tysk, German deutsch, Dutch duits < PG *þeudiskas. This implies that c /ts/ and ć /t͡ɕ/ may be an analogical reflection of the same word, except where PBS has *k it might imply PS *kelawaikas (compare Czech), from *(s)kʷel- “crowd, people”, from *kʷel-, whence also Greek tele- ("far"). Similar arguments hold for Allemagne and Sasse. I mean it's speculative.
3: Anyway, Polish mówić is not exactly related to niemeć (en.Wiktionary; cf. Vasmer, Trubachev).