On p. 967 of "The Torah a Modern Commentary" (Plaut, revised edition) it is written, "...all of Israel's first born sons belong to G-d (Num. 8:13-17). Because G-d accepts the Levites as substitutes, they are set aside in a special way."
My question is, if G-d accepted the Levites in place of the male first born, then why do we need to redeem our male first born children today through the practice of Pidyon Haben?
13 וְהַֽעֲמַדְתָּ֙ אֶת־הַלְוִיִּ֔ם לִפְנֵ֥י אַהֲרֹ֖ן וְלִפְנֵ֣י בָנָ֑יו וְהֵנַפְתָּ֥ אֹתָ֛ם תְּנוּפָ֖ה לַֽיהֹוָֽה׃
You shall place the Levites in attendance upon Aaron and his sons, and designate them as an elevation offering to the LORD.
14 וְהִבְדַּלְתָּ֙ אֶת־הַלְוִיִּ֔ם מִתּ֖וֹךְ בְּנֵ֣י יִשְׂרָאֵ֑ל וְהָ֥יוּ לִ֖י הַלְוִיִּֽם׃
Thus you shall set the Levites apart from the Israelites, and the Levites shall be Mine.
15 וְאַֽחֲרֵי־כֵן֙ יָבֹ֣אוּ הַלְוִיִּ֔ם לַעֲבֹ֖ד אֶת־אֹ֣הֶל מוֹעֵ֑ד וְטִֽהַרְתָּ֣ אֹתָ֔ם וְהֵנַפְתָּ֥ אֹתָ֖ם תְּנוּפָֽה׃
Thereafter the Levites shall be qualified for the service of the Tent of Meeting, once you have cleansed them and designated them as an elevation offering.
16 כִּי֩ נְתֻנִ֨ים נְתֻנִ֥ים הֵ֙מָּה֙ לִ֔י מִתּ֖וֹךְ בְּנֵ֣י יִשְׂרָאֵ֑ל תַּ֩חַת֩ פִּטְרַ֨ת כׇּל־רֶ֜חֶם בְּכ֥וֹר כֹּל֙ מִבְּנֵ֣י יִשְׂרָאֵ֔ל לָקַ֥חְתִּי אֹתָ֖ם לִֽי׃
For they are formally assigned to Me from among the Israelites: I have taken them for Myself in place of all the first issue of the womb, of all the first-born of the Israelites.
17 כִּ֣י לִ֤י כׇל־בְּכוֹר֙ בִּבְנֵ֣י יִשְׂרָאֵ֔ל בָּאָדָ֖ם וּבַבְּהֵמָ֑ה בְּי֗וֹם הַכֹּתִ֤י כׇל־בְּכוֹר֙ בְּאֶ֣רֶץ מִצְרַ֔יִם הִקְדַּ֥שְׁתִּי אֹתָ֖ם לִֽי׃
For every first-born among the Israelites, man as well as beast, is Mine; I consecrated them to Myself at the time that I smote every first-born in the land of Egypt.