Nominative, vocative, accusative, instrumental, dative and genitive seems like a perfectly good case inventory to me. Cases often drop put. PIE had eight, but even today there are Indo-European languages which retain all of them.
Specificity, definiteness and animacy all correlate positively to the use of "the accusative a". Which combinations of these result in the preposition being used depend on the dialect.
Actually, one can ask the same question about the Turkish example:
Ben Sam ile filmi görüyoruz.
Ben Sam with film-ACC watch-PROG-1PL
Ben and I are watching the film.
Ben Sam ile film görüyoruz.
Ben Sam with film watch-PROG-1PL
Ben and I are watching a film.
Notice the -i suffix dropping when the object is indefinite.
Turkish and Spanish are far from alone in this. There are something like three hundred other languages where similar circumstances matter.
https://en.wikipedia.org/wiki/Different ... ct_marking < Source
Prepositions don't like to attach to their nouns and become prefixes for whatever reason. Postpositions do, but they are often hindered. There are often attributive adjectives, demonstratives or numerals in the way and even then a huge majority of languages put their relative clauses after the head and the postposition ends up far from its noun.
In those few lucky
cases where everything lines up, like in Japanese, there may seem to be many cases though there aren't.
You think <neko wo> means "cat" in the accusative case but then you find something like <neko dake wo>, "only the cat" and <wo> appears more like a postposition.
Personally, I think everything has lined up for Spanish so that it eventually will have case prefixes.
Sometimes, cases stack to form new ones. So Proto-Uralic had inessive -s, partitive -ta and essive -na. Soon they were combined like this: -sta, -sna. That's where Finnish got -sta(, out of,) and a new inessive case, -ssa, from. 3 cases became 4.
Also, I didn't know that Iyionaku was prone to illeism.