Because they don't exist in the tables
org-entities-user. You can customize the latter and add them if you want.
You need to figure out what to replace it with for LaTeX, HTML, ASCII, Latin1 and UTF8. Here's a possible definition for
\bot although the Latin1 part could probably be improved:
'("bot" "\\bot" t "⊥" "_|_" "_|_" "⊥"))
The doc for the variable says:
User-defined entities used in Org to produce special characters.
Each entry in this list is a list of strings. It associates the name
of the entity that can be inserted into an Org file as \name with the
appropriate replacements for the different export backends. The order
of the fields is the following
name As a string, without the leading backslash.
LaTeX replacement In ready LaTeX, no further processing will take place.
LaTeX mathp Either t or nil. When t this entity needs to be in
HTML replacement In ready HTML, no further processing will take place.
Usually this will be an &...; entity.
ASCII replacement Plain ASCII, no extensions.
Latin1 replacement Use the special characters available in latin1.
utf-8 replacement Use the special characters available in utf-8.
If you define new entities here that require specific LaTeX
packages to be loaded, add these packages to ‘org-latex-packages-alist’.