Question:  Why does Hebrews 10:25 seem to indicate that regular church attendance is mandatory for all believers?

Answer:   It does not.  Hebrews 10:25 in the AV7 Bible reads:  "Let us not forsake the assembling of ourselves together,  as the manner of some is,  but exhort one another.  So much the more,  as you see the day approaching."

This verse of scripture is expository preaching/teaching by an unknown writer.

It is not a commandment,  not a law,  not a statute,  not an ordinance,  and not even an instruction.  It does not mandate obedience,  per se.  It is also important to keep in perspective that it is not a word spoken  "out of the mouth of God."  Matthew 4:4 clearly defines the biblical criteria for "rightly dividing the Word of Truth" to distinguish the literal "Word of God" (Revelation 19:13) from the rest of the accompanying Scripture in the Bible.

Rather,  this verse in Hebrews 10:25 simply encourages believers to "assemble" together for fellowship and to "exhort" (or encourage) one another to do so.  It is also appropriate to note that the word "church" does not appear in this verse.

Therefore, this verse cannot be claimed to either insist or demand or mandate or require "church" attendance,  per se.

One of the problems that has plagued the Christian faith throughout history has been the tendency of many preachers to extrapolate the plain words in the Bible into man-made rules and doctrines that impose requirements far beyond what the scriptures actually teach.  Faith in God and in Jesus Christ is much less complex,  cumbersome,  and demanding than what most organized religions try to make it out to be.

Consider Jesus' words:  Those who believe and are baptized will be saved,  but those who do not believe will be damned.  -Mark 16:16.  And:  Whoever therefore will confess me before people+,  I will confess also before my Father in heaven.  -Matthew 10:32.  Faith in God really is just that simple.

Also consider this simple biblical principle:  If you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead,  you will be saved.  -Romans 10:9.  (Also note the future tense of "will be saved.")

Church attendance can be very beneficial.  Or,  it can be unhelpful if and when man-made doctrines are taught and inappropriately imposed as man-made requirements.  cf. Luke 11:42-43.