Supplementing @GershonGold's answer, see this article. There is a controversy as to whether the brachot recited before the haftarah are really 1 or 2 brachot.
In response to your second question, there are a few reaons, all based on what I am inferring form the linked article.
Haftarah was originally considered supplemental to the Torah reading, if you view the history mentioned in this article . It was only later, that it temporarily became a substitute for the Torah reading. I'm surmising, then, that since it's "attached" to the Torah reading, it's not considered a "separate" mitzvah.
Citing excerpts from the linked article who quotes Avudraha"m:
First we say: Asher Bachar B’Nevim Tovim meaning that we are excluding
false prophets.. Chazal decided to show honor to the Torah and to
Moshe, teacher of all the Prophets and then added that G-d chose the
Torah and Moshe his servant. In order not to make it appear that Moshe
was being mentioned after the Prophets, Chazal began the second part
of the Bracha with the word: Baruch but not to indicate that it was
the beginning of a separate Bracha. If that was Chazal’s intention,
Chazal would have closed with a Bracha and not opened with a Bracha.
Since Chazal mention the Torah and Moshe they returned to mentioning
the Prophets who were true and to mention what was above in a place
I'm surmising that the first words in the 1st bracha start with Asher Bachar which, while not indicating, directly, that it is a mitzvah to recite haftarah, seems to indirectly imply that since G-d chose truthful prophets whose words are no less important then those of Moshe in the Torah, in a sense, we are obliged to mention the words of the prophets as well as the words of the Torah.