「彼{かれ}ら は 自分{じぶん} の 事{こと} しか 考{かんが}えて いません。」
While the translation:
"They only think of themselves."
is a valid one in the sense that it successfully conveys the basic meaning of the original, it can also be highly misleading as far as the grammatical understanding of the original.
The original Japanese sentence is indeed in the negative form even though the English translation I just called 'valid' is clearly in the affirmative form.
The key word is 「しか」, which is always used in conjunction with a negative verb phrase.
「Noun + しか + Verb in Negative Form (Verb + ない/ません, etc.)」
Literally means:
"to not [Verb] anything but [Noun]"
Therefore,
「自分のことしか考えていません」
literally means:
"not thinking about anything except for themselves"
which in turn means:
"to only think about themselves"
The less wordy translations, therefore, tend to be used more commonly over time.