Hello there - I'm not an historian, so I may be breaking the rules by posting. Then again, I don't think this is really an historical question.
The reason the poem rhymes in English, despite being written in Latin, is that the translator has decided to make it that way - and probably worked hard to do it.
A common misconception about translation is that it's mechanistic or deterministic - given a work in, say, French, there's a single correct translation of that work into English. But that isn't true - take the sentence "Je marche". What's the correct translation? "I walk"? "I am walking"? Either could be right - maybe the context makes it clear, but if not, it's up to the translator to choose.
Things get even stickier with poetry - what should the translator keep the same? The meaning? The rhyme scheme? The line lengths? It usually won't be possible to do all of these. This is true even for simple poems - compare the French song Frere Jacques with its most common English translation, where the first two lines have been swapped, and the meaning of the third changed, in order to keep the musical quality of the poem.
For a fascinating look at these issues (plus AI, music, love, and more), see Douglas Hofstadter's book Le Ton Beau de Marot, which has about thirty translations of a single short poem by Marot, all different, but all capturing some aspect of the original.
Hey there! Could you help us out a little and let us know what translator/edition/publisher of Lucretius you are using?
/u/SirVentricle was talking recently about the art of translation (in general - not re this translation specifically), which may be of some interest here