In Sefer Devarim, Moshe Rabbeinu, in his last speech to the Jewish people, seems to constantly tell the Jewish people that they themselves witnessed the miracles and G-dly revelations. For example:
But beware and watch yourself very well, lest you forget the things that your eyes saw, and lest these things depart from your heart, all the days of your life, and you shall make them known to your children and to your children's children,
רַק הִשָּׁמֶר לְךָ וּשְׁמֹר נַפְשְׁךָ מְאֹד פֶּן תִּשְׁכַּח אֶת הַדְּבָרִים אֲשֶׁר רָאוּ עֵינֶיךָ וּפֶן יָסוּרוּ מִלְּבָבְךָ כֹּל יְמֵי חַיֶּיךָ וְהוֹדַעְתָּם לְבָנֶיךָ וְלִבְנֵי בָנֶיךָ:
10 the day you stood before the Lord your God at Horeb, when the Lord said to me, "Assemble the people for Me, and I will let them hear My words, that they may learn to fear Me all the days that they live on the earth, and that they may teach their children.
יוֹם אֲשֶׁר עָמַדְתָּ לִפְנֵי יְהֹוָה אֱלֹהֶיךָ בְּחֹרֵב בֶּאֱמֹר יְהֹוָה אֵלַי הַקְהֶל לִי אֶת הָעָם וְאַשְׁמִעֵם אֶת דְּבָרָי אֲשֶׁר יִלְמְדוּן לְיִרְאָה אֹתִי כָּל הַיָּמִים אֲשֶׁר הֵם חַיִּים עַל הָאֲדָמָה וְאֶת בְּנֵיהֶם יְלַמֵּדוּן:
Or Devarim 7:19:
The great trials that your eyes saw, the signs, the wonders, the mighty hand, and the outstretched arm with which the Lord, your God, brought you out. So will the Lord, Your God, do to all the peoples you fear.
הַמַּסֹּת הַגְּדֹלֹת אֲשֶׁר רָאוּ עֵינֶיךָ וְהָאֹתֹת וְהַמֹּפְתִים וְהַיָּד הַחֲזָקָה וְהַזְּרֹעַ הַנְּטוּיָה אֲשֶׁר הוֹצִאֲךָ יְהֹוָה אֱלֹהֶיךָ כֵּן יַעֲשֶׂה יְהֹוָה אֱלֹהֶיךָ לְכָל הָעַמִּים אֲשֶׁר אַתָּה יָרֵא מִפְּנֵיהֶם:
However, from Bamidbar 26:63-65 we see that all the censused men who left egypt had already died, and this was a new generation:
This was the census of Moses and Eleazar the kohen, who counted the children of Israel in the plains of Moab, by the Jordan at Jericho.
אֵלֶּה פְּקוּדֵי משֶׁה וְאֶלְעָזָר הַכֹּהֵן אֲשֶׁר פָּקְדוּ אֶת בְּנֵי יִשְׂרָאֵל בְּעַרְבֹת מוֹאָב עַל יַרְדֵּן יְרֵחוֹ:
Among these there was no man who had been [included] in the census of Moses and Aaron when they counted the children of Israel in the Sinai desert.
וּבְאֵלֶּה לֹא הָיָה אִישׁ מִפְּקוּדֵי משֶׁה וְאַהֲרֹן הַכֹּהֵן אֲשֶׁר פָּקְדוּ אֶת בְּנֵי יִשְׂרָאֵל בְּמִדְבַּר סִינָי:
For the Lord had said to them, "They shall surely die in the desert," and no one was left of them but Caleb the son of Jephunneh and Joshua the son of Nun.
כִּי אָמַר יְהֹוָה לָהֶם מוֹת יָמֻתוּ בַּמִּדְבָּר וְלֹא נוֹתַר מֵהֶם אִישׁ כִּי אִם כָּלֵב בֶּן יְפֻנֶּה וִיהוֹשֻׁעַ בִּן נוּן:
Now we know that anyone who was younger than 20 at the time of the first census did make it into the Promised Land (Bamidbar 14:29). It is understood from there that anyone who was above 60 at the time of the first census also made it into the land. The women also didn't die in the desert, nor did the Levites.
All that notwithstanding, ~600,000 men had died and were replaced with a new generation, at least half of which had not been born at the time of the exodus (anyone of the censused men between 20 and 40 at the time of Moshe's speech would have been born afer the exodus from Egypt).
If so, why did Moshe tell them all that they personally witnessed the events such as the giving of the Torah?
It seems weak to answer that Moshe was talking to the majority, even though a substantial portion had not themselves witnessed the events.