Replacement iPhone 4 Display Assembly Parts

So I keep seeing responses to posts about having to purchase the digitizer, frame, glass and LCD as one unit for an iPhone 4. However I only see these responses for questions asked about only replacing one of those items. For example a person has a broken front glass but the digitizer is fine, or they only want to replace the LCD component. My question is this,
As many of you likely know, eBay has a plethora of offerings for a person to buy the glass, digitizer, frame and LCD units separately. What I am confused about is why I wouldn't be able to purchase all the parts separately to make my own Display Assembly.
No one has really clearly pointed out why you can't combine all the parts yourself to make the assembly.
I understand that the LCD and Digitizer are fused together with the OEM part.
If they need to be fused together, then why? The way I see it, I could buy the frame, glass and digitizer as one unit and the LCD as a separate unit and just slap them together (not fusing them). I am sure I will have issues with dust and debris possibly getting between the front and the LCD, but shouldn't the parts function under this presumption? Isn't this how previous iPhone models worked...?
I look forward to hearing any and all responses to these questions.
Thanks for stopping and reading my post.

Update

To those who might browse this topic.
Alex Jansen and Scott Head wrote back confirming that in theory, using a separately purchased digitizer assembly (glass, frame & digitizer) and an LCD unit should function as expected, in reference to my questions about the front glass, digitizer and LCD components.
So to clarify (for those of you with a similar question),
If you wanted to purchase the iPhone 4 Display Assembly components separately, you could and they should work, how well though is a different story all together.
I have yet to see a concrete write-up or video of someone who has taken this course of action.
If you've got a link you could share, showing a video of someone fully disassembling an iPhone 4 and replacing the front assembly with a non-fused LCD panel (or something close), that would be great.
I have also included a link below to a YouTube video of someone who leads you to believe the LCD unit is separate, but the video skips the part showing the LCD placed in the digitizer.
YouTube Video Link