The object is more important than the plural subject
It's not about who gives the defense, but about whom the defense is given to
You're asking about the phrase:
[you] (plural) ready to give a defense (singular)
Meaning of distributive vs collective
Distributive plural (MLA)
When each part of a plural subject possesses something individually, the thing possessed must generally be in the plural as well. For example:
The two women blew their noses.
Collective plural (Univ of S Carolina)
This is the collective reading...
The men lifted the piano.
The phrase in Greek means the idea of the distributive plural, even though the noun is singular, reading like this:
The two women blew their nose.
In English, we're all worried up the kazoo about making these match perfectly. So, we're naturally asking English questions that the Greek speakers naturally didn't care or think about.
Koine Greek is slang and not necessarily that precise in matching the number for subjects with verbs. Peter didn't have to follow the "Greek MLA stylebook" in his day because there wasn't one. So, he not trying to be as technical as we would be in English.
He's just addressing the group in the plural, then talking about individual situations in the singular.
1 Peter 3:15 (SBLG)
κύριον δὲ τὸν Χριστὸν ἁγιάσατε ἐν ταῖς καρδίαις ὑμῶν, ἕτοιμοι ἀεὶ πρὸς ἀπολογίαν παντὶ τῷ αἰτοῦντι ὑμᾶς λόγον περὶ τῆς ἐν ὑμῖν ἐλπίδος,
but sanctify Lord Christ in your hearts, always ready with an answer for everyone who requests you account for the hope in you
lit: (emphasized segment)
Objects and slang grammar
Sometimes "bad" grammar makes the point better. But, Peter didn't have "bad" grammar because there wasn't a "Greek MLA stylebook" in his day. They just used the words that best made the point.
[you (plural)] always ready with an answer is akin to saying:
All of you, search your heart.
That's technically incorrect, but we understand the "correct" meaning to be:
Each of you, search your [own] heart.
The grammatically incorrect way is less wordy and easier to read, say, and hear. That's why we are all guilty of doing this all the time in English. That's what Koine Greek does, albeit they didn't have grammar style books to say it was "wrong", but this was how Koine Greek was used.
This each of you type of language follow "everyone who requests you" part of the verse.
Note, with everyone who and requests you, I consider both to be raised objects, which helps this make more sense. This is like saying, "I saw the dog eat the rabbit eat the carrot," where dog and rabbit are raised objects. Peter is talking this way in ready with an answer for everyone who requests you account.
Peter's point is for the individual
The emphasis is on each individual asking for an account, seeking an answer. The term "every one" is a plurality, but treated as individuals, so this will also be treated as singular, or a "multiple singular".
Everyone, please take your seat.
The speaker is addressing many people, but as individuals.
This would be grammatically wrong in English:
Everyone, please take your seats.
Essentially, Peter makes "a defense/answer" singular, not because all Christians should only have one, singular, group-approved answer for all visitors, but because every individual who asks deserves an individual, tailored answer.
Albeit, Peter does address them as a group, so the ability to do this is indeed a shared responsibility. But, it's not the same as saying:
All men, move the piano.
But, more like saying:
All men, move every piano.
It does read, "everyone who asks," after all.
There are multiple, individual jobs to do, and a plurality of people ready to do the jobs, and somehow they need to make sure that each individual job gets done. Whether every piano (and every answer) requires many people or just one isn't something Peter specifies. We're supposed to figure that out together and get each and every piano moved.
And, we are to give each and every person a good answer to whatever question is asked of any of us.