Your text editor has a dictionary that it consults in order to decide whether to complain about a word you type. This dictionary has an entry for endpoint but not for startpoint.
Why is that? It's hard to be sure without knowing which text editor you are using and where its dictionary comes from, but it is likely that one of the techniques used to compile the dictionary was to consider the most frequently used words in some corpus. And endpoint is a more frequently used word than startpoint, so it is more likely to be included in such a dictionary.
The OED notes that startpoint is rare:
start-point n. rare = starting-point n.
1876 J. Ruskin Fors Clavigera VI. lxii. 59, I find myself..without any start-point for attempt to understand them.
And here's a chart from Google Ngram Viewer showing their relative frequencies:
