To use your example from Japanese: ika-se-rare-ta-kuna-katta Yes, all one word. A word of 10 syllables, 6 morphenes. What, precisely, is the advantage over "didn't want to be made to go" (8 syllables, and 8 morphenes)? It only works if you decide that "word" is the most basic unit to measure against, but "word" can be a very arbitrary concept.
In a 2010 paper, a study was done looking at the "information density" of speech; that is, given the same text translated into multiple languages, so everyone is conveying the same information, how long would it take a fluent speaker of that language to transmit that information, speaking at a normal cadence (and using multiple speakers to get an average, of course). As part of that process, one step was determining how much information was conveyed per syllable. They used Vietnamese as a baseline, arbitrarily giving it a value of 1.00. If a language conveyed the same information in more syllables than Vietnamese, it would be lower than 1 (you need more syllables to communicate the same information). English was 0.91. Japanese was 0.49. In other words, you needed nearly twice as many syllables to communicate the same thing in Japanese as you did in English, Mandarin (0.94), or obviously Vietnamese (1.00).
When you compare speaking rates (how fast those syllables are spoken), it turns out that most languages are about the same when it comes to transmitting information per time, between 0.9 and 1.1 (Vietnamese again being 1.00). Except Japanese, which although having easily the fastest speaking rate of the languages tested, because of the extremely low density of information per syllable, they had the lowest rate of transmission by far, at 0.74.
So Japanese is significantly less information dense than many other widely spoken languages.
You see the same thing in other agglutinative languages. Just because a given language can pack more information into a single word doesn't mean that it will necessarily be shorter to communicate. I work with Inuktitut translations all the time, and I just pulled up an example I've dealt with:
The Proponent intends to conduct a mineral exploration program including drilling, sampling, magnetic surveying, and mapping.
The Inuktitut translation is:
ᐱᓕᕆᔪᒪᔪᖅ ᐅᔭᕋᖕᓂᐊᕐᕕᒃᓴᖅᓯᐅᕈᒪᒐᒥ ᐱᓕᕆᓇᔭᖅᓱᓂ ᐃᑰᑕᖃᕐᓂᖅ, ᐲᔭᐃᖃᑕᕐᓂᖅ ᖃᐅᔨᓴᒐᒃᓴᓂᒃ, ᓇᐅᒃᑯᑦ ᓂᐱᖓᓂᖃᕐᓂᖏᓐᓂᒃ ᓄᓇᐅᑉ ᐃᓗᐊᒍᑦ ᐅᔭᕋᖏᑦ ᐊᒻᒪᓗ ᓄᓇᖑᓕᐅᓂᕐᒥᒃ.
Which transliterates as:
Pilirijumajuq ujarangniarviksaqsiurumagami pilirinajaqsuni ikuutaqarniq, piijaiqatarniq qaujisagaksanik, naukkut nipinganiqarninginnik nunaup iluagut ujarangit ammalu nunanguliunirmik.
Obviously the English is much more concise simply based on inspection even though it has more words. Now, an argument can be made that the English has some terms which might require a longer word to explain it in Inuktitut, so that's a fair argument. So here is a more "common language" example from the Government of Nunavut's website (https://livehealthy.gov.nu.ca/en/health-topics/injuries/preparing-hunt-land):
Telling someone at home where you are going. Even if you are going
out on the land for just a few hours, you should tell at least one
person at home the names of all riders and passengers, where you are
going, and when you plan to come back. If you do not return in time,
this person will be able to send for help.
And here is the Innuinaqtun version:
Uqaqlugu kimut humungauliqtutin. Aulaaqhimanahuaqtillutin ikituni
ikaangnini, uqaqtukhauyutin atauhinaugumi inuk aimavingni kitkut
aulaaqatiniatatin, humungauliktutin, humi utiqnahuaqtuninlu.
Uttinngitkuvin mikhaatigun, tamna inuk ikayutikhangnik
The Innuinaqtun is 21 words compared to the English 67. If, however, you look at the syllables, there's 109 compared to English's 73. English clearly has the higher information density per syllable. Just because the average word is longer doesn't make the language more information dense.
Part of the issue with Inuktitut, just as with Japanese, is that the number of possible different syllables is limited due to the phonotactics of the language, which means you have to use more syllables to compensate for the limited inventory you have. Standard Inuktitut has at most 210 possible syllables. Japanese has, depending on who you ask, about 400 possible syllables, so you could have that number of possible one-syllable words. With English phototactics, for "standard" English, you have about 316,800 possible one-syllable words (there is about 9,300 in use).
The reason for this difference is the number of consonant clusters allowed and the ridiculous number of vowels in most English dialects.
tl;dr summary: to have a more information dense and still practical language (so you allow some natural redundancy and not have so many near-homonyms that mininterpretation due to missing something is a constant problem), you need a language with a lot of possible syllables and phonotactics that allow those syllables to be used or combined into sufficiently many distinct morphenes, however you put those morphemes together or not in any kind of language, whether isolating or agglutinative.