Translation:I don't like to learn long things by heart.
Good question. There's not any principled reason, and there could have been a transitive root for 'memorize', but the root parker- is intransitive and actually means something like 'know by memory/heart' (it's derived from French par cœur). So you can say Mi scias tion parkere 'I know that by heart'.
That would be an idiomatic expression to call memorising something completely knowing it “by heart” (per koro). :D That would sure make sense for an English speaker or a speaker of some Romance languages, but that's not the way to go in Esperanto.
Zamenhof decided that there's a need of the word for knowing “by heart, thoroughly” (par cœur | auswendig | наизусть | na pamięć), rather than to have it constructed with already existing vocabulary (although en·memor·ig·i works perfectly fine for the same concept) and so there's this root parker-.
The root parker- has intrinsically a descriptive nature (rather than action/verbal or thing/nominal one) and so in dictionaries you’ll find its meaning given first as an adverb parker·e. Therefore we need the -ig- suffix, to say that we’re making something have this particular property (of being known by heart).
The verb parker·i isn’t used at all, but if it were, it would have mean est·i parker·a. The adjective parker·a is used rarely and means “learned by heart, related to knowing by heart” and so either the poem can parker·i in your memory, or your knowledge of this poem can parker·i. Not very useful, but that should be the meaning if one would ever use this verb. :)