This a more conceptual than technical answer to explain why not, that need the context of previous questions:
Why the captions and pages are almost always numbered?
Well, there are two possible answers. Because ...
1) The stupid habit of numbering everything for no reason.
2) This help to the reader when you write references as "see figure 3 on page 5", for instance. Hoping we are rationals, let choose the second, but ...
Why these text references to numbered captions help to the reader?
Because often the figures and tables are far away from their context, that is, when some passage make this figure or table relevant.
Then, why not put the figures and tables exactly when is relevant, in a ""see next figure:" style, with the float with
[H] just in the place of the text reference?
I agree completely with Ulrike that referencing plots in this way, then use
[H] have sense, and there are texts and texts. but IMHO at least in formal texts (thesis, scientific journals, etc.) this way should be avoided by several reasons:
1) Because always disrupt the narrative.
Specially if there are a caption in the middle of text. Any good essay could have tables and figures supporting the meaning of the text with additional information, but the text alone should be fully understandable without this supplement. It should be the reader who will have to decide if he needs stop the lecture in this moment to scrutinize a graphic to collect more information about you have wrote, (or worse, clarify what are you explaining) or before read the whole article, or after. At least in scientific journals, the philosophy behind is the same that prescribing the use of abstracts, keywords and self-explicative captions: help to the reader. Options
p are always better in this sense.
2) Because very often is not possible stuck tables and figures to relevant text.
One table could be relevant for several passages of different pages. On the other hand, only one passage could be enough explanation for several tables and figures that must be in different pages. Obviously the solution is not make duplicates of a table each time that worth mention it, or make introductory paragraphs for each figure, but make cross-references.
The other two have been pointed by David but I want insist on it:
3) Because can disrupt also the layout.
This is most important technical disadvantage IMHO. If some non floating box does not fit at the end of the page, this imply a premature page break leaving an huge, ugly and baffling space ("What next figure? there are nothing below" or "Why there are no more text? It is the end of the section?", etc. ). A float with
[!ht] probably will go away from their context nearly the same but without ruin the previous page layout. If you are unsure if the float will fit or not [!ht]
4) Because forcing floats positions probably can cause troubles in another floats.
Typical questions "why my float go to the end of the document?" are caused by the obsession of control float positions. As more flexibility you left, more near will be the floats of the original surrounding text and better balance between amount of text and floats per page without having to move manually the float positions.