How do you look up a conjugated word in a dictionary?
Dictionaries are organised by headword — the infinitive of a verb, the singular of a noun. Text is not. You read vécut, mangeaient, wended or oxen, and none of those is the entry you need. Working out the root before you can search is an extra step, and in a language you are still learning it is the hard step.
Some dictionaries handle this and many do not. Wiktionary generally recognises inflected forms; plenty of dictionary apps do not, and simply return nothing. If you are choosing a tool for reading rather than for writing, this is the single capability worth testing first.
Booktionary performs this reduction before it searches, so vécut resolves to vivre, mangeaient to manger, were to be and mice to mouse. It is the reason it works on novels rather than only on menus and signs.
For French specifically, the passé simple is the form that catches most learners out, because it appears constantly in literature and almost never in speech.