Those abbreviations are regularly used in dictionaries (and similar publications) because there is limited space. However, we like all words to be written out in full on the site because it is less confusing to beginners. Learners on the site who are not familiar with these definitions might be completely flummoxed by the use of "sb" and "sth" - they might try to look those up as proper words. Even when they know that they are abbreviations, they still might to try and use them in normal writing. Some abbreviations are perfectly acceptable in standard writing ("etc" for example) but "sb" and "sth" are not.