IFilters are often provided free by the same people who make the document formats or editors for them. It makes sense for them to provide a way to extract the text from their documents, rather than every other piece of software having to have its own custom code to decode every format. They are the experts in their own formats, and the code only needs writing once, by them, if it's turned into an IFilter which everything else can use.
Have you looked for one and found they all cost money or is that just an assumption? (I do not know either way as I don't use OpenOffice and it hasn't come up recently if ever before. A quick search of this forum shows one previous mention which links to a free one but there may be better/newer ones if you search more relevant sites for OpenOffice add-ons.)