"Than" is used when comparing two objects. The basic forms are "more X than Y" and "less X than Y." Susan is more helpful than Anna. Some verbs inherently make a comparison and the words "more ... than" and "less ... than" aren't used. Joe outperforms Sam. Sam underperforms Joe. The Oxford Learner's Dictionaries website is an excellent resource for learners. outperform verb https://www.oxfordlearnersdictionaries.com/definition/english/outperform?q=outperform ​outperform somebody/something to achieve better results than somebody/something The company has consistently outperformed its larger rivals.

Chris is more helpful than I am. Chris outperforms me :(

Thank you both of you!! I totally get it now. :D

“than” can only be used with adjectives or adverbs while “outperform” is a verb. You might want to check out the word’s definition here so that you can see it was used correctly in the first sentence: https://www.google.com.vn/amp/s/dictionary.cambridge.org/amp/english/outperform

The use of the prefix "out-" acts as the comparative in place of "than" in this example. If the prefix were not there, it would be written as "performed better than those who..". You can add the prefix "out-" to most verbs to imply that the formerly mentioned group/person is better than the latter.