Why is some information from the Wikipedia infobox missing on DBpedia?
For example, star Alpha Librae has property distance-from-earth in the infobox, but it isn't a property of the Alpha Librae dbpedia resource. On the other hand, star Betelgeuse has this piece of information on DBpedia). And many other stars have this distance information in the infobox, but there isn't any matching property in the DBpedia resource.
Is there a way to extract thise missing information from DBpedia using SPARQL or is the only way web scraping of the wiki page?
The DBpedia pages present all the data DBpedia has -- no SPARQL nor other query can get data that isn't there.
DBpedia is updated periodically. It may not reflect the latest changes on Wikipedia.
Also, extractors are a living project, and may not grab every property in which you're interested.
Looking at Betelgeuse on Wikipedia, I see one distance in the infobox. Looking at Alpha_Librae, I see two distances. Which should DBpedia have? Perhaps you have the niche knowledge which can ensure that the extractors do the right thing...
As @JoshuaTaylor suggests, you will probably get more satisfactory answers from the DBpedia discussion list and/or the DBpedia development list.