Why doesn't​ the Torah mention the Beis Hamikdash?

Question

Only the Mishkan is mentioned, which was temporary.

Answer 1

Shmot 25.8:

"וְעָשׂוּ לִי מִקְדָּשׁ וְשָׁכַנְתִּי בְּתוֹכָם׃"

And let them make Me a sanctuary that I may dwell among them.

Rambam brings it to Halachah (Beit haBchira 1.1)

מצות עשה לעשות בית ליי' מוכן להיות מקריבים בו הקרבנות וחוגגין אליו שלש פעמים בשנה שנאמר ועשו לי מקדש וכבר נתפרש בתורה משכן שעשה משה רבינו והיה לפי שעה שנאמר כי לא באתם עד עתה וגו':

It is a positive commandment to construct a House for God, prepared for sacrifices to be offered within. We [must] celebrate there three times a year, as [Exodus 25:8] states: "And you shall make Me a sanctuary

In short, the Mikdash was a continuation of the Mishkan, and all the commandments of the later apply to the Mikdash also (in general). The prophets (incl. David and Salomon) saw some changes and upgrades, but the idea remained.

Answer 2

We find several references in the Torah to a “place where Hashem will choose to dwell His Name there”; as this is in the future tense, and in Sefer Devarim - i.e. long after the Mishkan was built - it must refer to the Beis HaMikdash. Some even use phrases like “amongst your tribes” and “when you come to the land,” which indicate that it’s in Eretz Yisrael.

12:5:

כִּ֠י אִֽם־אֶל־הַמָּק֞וֹם אֲשֶׁר־יִבְחַ֨ר יְהוָ֤ה אֱלֹֽהֵיכֶם֙ מִכָּל־שִׁבְטֵיכֶ֔ם לָשׂ֥וּם אֶת־שְׁמ֖וֹ שָׁ֑ם

Except to the place which Hashem your G-d will choose from amongst your tribes to rest His Name there

12:11:

וְהָיָ֣ה הַמָּק֗וֹם אֲשֶׁר־יִבְחַר֩ יְהוָ֨ה אֱלֹהֵיכֶ֥ם בּוֹ֙ לְשַׁכֵּ֤ן שְׁמוֹ֙ שָׁ֔ם

And it will be the place which Hashem your G-d will choose in it to dwell His Name there

12:14:

כִּ֣י אִם־בַּמָּק֞וֹם אֲשֶׁר־יִבְחַ֤ר יְהוָה֙ בְּאַחַ֣ד שְׁבָטֶ֔יךָ

Except in the place which Hashem will choose in one of your tribes

12:18:

בַּמָּקוֹם֙ אֲשֶׁ֨ר יִבְחַ֜ר יְהוָ֣ה אֱלֹהֶיךָ֮ בּוֹ֒

In the place which Hashem your G-d will choose it

12:21:

כִּֽי־יִרְחַ֨ק מִמְּךָ֜ הַמָּק֗וֹם אֲשֶׁ֨ר יִבְחַ֜ר יְהוָ֣ה אֱלֹהֶיךָ֮ לָשׂ֣וּם שְׁמ֣וֹ שָׁם֒

If the place which Hashem your G-d will choose to rest His Name there is too far from you

12:26:

וּבָ֔אתָ אֶל־הַמָּק֖וֹם אֲשֶׁר־יִבְחַ֥ר יְהוָֽה׃

And you will come to the place which Hashem will choose

14:23-25:

וְאָכַלְתָּ֞ לִפְנֵ֣י ׀ יְהוָ֣ה אֱלֹהֶ֗יךָ בַּמָּק֣וֹם אֲשֶׁר־יִבְחַר֮ לְשַׁכֵּ֣ן שְׁמ֣וֹ שָׁם֒ [...] כִּֽי־יִרְחַ֤ק מִמְּךָ֙ הַמָּק֔וֹם אֲשֶׁ֤ר יִבְחַר֙ יְהוָ֣ה אֱלֹהֶ֔יךָ לָשׂ֥וּם שְׁמ֖וֹ שָׁ֑ם כִּ֥י יְבָרֶכְךָ֖ יְהוָ֥ה אֱלֹהֶֽיךָ׃ וְנָתַתָּ֖ה בַּכָּ֑סֶף וְצַרְתָּ֤ הַכֶּ֙סֶף֙ בְּיָ֣דְךָ֔ וְהָֽלַכְתָּ֙ אֶל־הַמָּק֔וֹם אֲשֶׁ֥ר יִבְחַ֛ר יְהוָ֥ה אֱלֹהֶ֖יךָ בּֽוֹ׃

And you will eat it before Hashem your G-d in the place where He will choose to dwell His Name there [...] If the place which Hashem your G-d will choose to rest His Name there is too far from you, for Hashem has blessed you, you may put it in money, and carry the money in your hand, and go to the place which Hashem your G-d will choose it.

15:20:

בַּמָּק֖וֹם אֲשֶׁר־יִבְחַ֣ר יְהוָ֑ה

In the place which Hashem will choose

16:2:

בַּמָּקוֹם֙ אֲשֶׁר־יִבְחַ֣ר יְהוָ֔ה לְשַׁכֵּ֥ן שְׁמ֖וֹ שָֽׁם׃

In the place where Hashem will choose to dwell His Name there

16:6-7:

כִּ֠י אִֽם־אֶל־הַמָּק֞וֹם אֲשֶׁר־יִבְחַ֨ר יְהוָ֤ה אֱלֹהֶ֙יךָ֙ לְשַׁכֵּ֣ן שְׁמ֔וֹ שָׁ֛ם [...] וּבִשַּׁלְתָּ֙ וְאָ֣כַלְתָּ֔ בַּמָּק֕וֹם אֲשֶׁ֥ר יִבְחַ֛ר יְהוָ֥ה אֱלֹהֶ֖יךָ בּ֑וֹ

Except to the place which Hashem your G-d will choose to dwell His Name there [...] And you will cook it and you will eat it in the place which Hashem your G-d will choose it

16:11:

בַּמָּק֗וֹם אֲשֶׁ֤ר יִבְחַר֙ יְהוָ֣ה אֱלֹהֶ֔יךָ לְשַׁכֵּ֥ן שְׁמ֖וֹ שָֽׁם׃

In the place where Hashem your G-d will choose to dwell His Name there.

16:15-16:

בַּמָּק֖וֹם אֲשֶׁר־יִבְחַ֣ר יְהוָ֑ה [...] שָׁל֣וֹשׁ פְּעָמִ֣ים ׀ בַּשָּׁנָ֡ה יֵרָאֶ֨ה כָל־זְכוּרְךָ֜ אֶת־פְּנֵ֣י ׀ יְהוָ֣ה אֱלֹהֶ֗יךָ בַּמָּקוֹם֙ אֲשֶׁ֣ר יִבְחָ֔ר

In the place which Hashem will choose [...] Three times a year will all your males be seen by the face of Hashem your G-d in the place which He will choose

17:8:

וְקַמְתָּ֣ וְעָלִ֔יתָ אֶל־הַמָּק֔וֹם אֲשֶׁ֥ר יִבְחַ֛ר יְהוָ֥ה אֱלֹהֶ֖יךָ בּֽוֹ׃

And you will get up and you will go up to the place which Hashem your G-d will choose it

17:10:

וְעָשִׂ֗יתָ עַל־פִּ֤י הַדָּבָר֙ אֲשֶׁ֣ר יַגִּ֣ידֽוּ לְךָ֔ מִן־הַמָּק֣וֹם הַה֔וּא אֲשֶׁ֖ר יִבְחַ֣ר יְהוָ֑ה

And you will do according to that which they tell you from that place which Hashem will choose

18:6:

אֶל־הַמָּק֖וֹם אֲשֶׁר־יִבְחַ֥ר יְהוָֽה׃

To the place which Hashem will choose

26:1-2:

וְהָיָה֙ כִּֽי־תָב֣וֹא אֶל־הָאָ֔רֶץ אֲשֶׁר֙ יְהוָ֣ה אֱלֹהֶ֔יךָ נֹתֵ֥ן לְךָ֖ נַחֲלָ֑ה וִֽירִשְׁתָּ֖הּ וְיָשַׁ֥בְתָּ בָּֽהּ [...] וְהָֽלַכְתָּ֙ אֶל־הַמָּק֔וֹם אֲשֶׁ֤ר יִבְחַר֙ יְהוָ֣ה אֱלֹהֶ֔יךָ לְשַׁכֵּ֥ן שְׁמ֖וֹ שָֽׁם׃

And it will be when you come to the land which Hashem your G-d gives to you as an inheritance, and you will inherit it and you will dwell there...and you will go to the place which Hashem your G-d will choose to dwell His Name there.

31:11:

בְּב֣וֹא כָל־יִשְׂרָאֵ֗ל לֵרָאוֹת֙ אֶת־פְּנֵי֙ יְהוָ֣ה אֱלֹהֶ֔יךָ בַּמָּק֖וֹם אֲשֶׁ֣ר יִבְחָ֑ר

When all of Israel comes to be seen by the face of Hashem your G-d in the place which He will choose