That’s because, presumably, you are coming from the standpoint of a native English speaker! I’ve had friends who were native Chinese speakers as well as of other languages, and they said they found English difficult to learn because despite what English speakers might think - the alphabet doesn’t really help you pronounce everything. For example - the word “enough.” There’s nothing in the phonetic alphabet that prepares you to pronounce that word!
Of course English has verb conjugation. Conjugation simply means the derived forms of a verb based on a variety of surrounding linguistic factors, like gender, tense, number, etc. Like the verb “to be” - saying “he was” is the past tense and that’s a form of verb conjugation, as are the variations “they were”, “I was”, “she was,” etc. English also has noun declension - a simple example is the difference between books and *book/i. A slightly more complex example would be he and *him/i or child and children.
I meant linguistically, as in linguistics experts have not come to that conclusion - most of them agree that the difficulty of a language learning is subject based upon the native language of the target and other factors. But even still everything you listed was subjective. Like memorization: If you already know Japanese and recognize many Chinese characters, learning Chinese will be easier for you than if you come into a Chinese class knowing no characters. (Even better, if you already know Vietnamese - Vietnamese borrows a lot of vocabulary from Chinese). If you speak a language that has inflection, learning another one with inflection will be easier than if you don’t. Conversely, if you already know a Romance language, learning another one will be easier. You alluded to that earlier when you said that learning Spanish would make it easier to learn Italian or French.
In addition, some people find memorization easier than phonetics, so memorizing the kanji/hanzi you need to use on a regular basis is easier for them, not to mention that some cultures have more of a tradition/heritage of memorization within schools (particularly Southeast Asian educational systems that rely on drilling). Obviously some English native speakers find it possible to do so!