I uses luajitlatex because it is faster than lualatex but it is not enabled by default. Why isn't it available by default settings?
Because LuaJIT is not available on all platforms.
LuaJIT “only” supports x86, x64,ARM, PPC, e500, and MIPS but TeXLive also supports SPARC.
Acutally I have asked the exact same question some time ago on the tlbuild mailing list. Here is the thread: https://tug.org/pipermail/tlbuild/2018q2/004212.html
I would like to note that support for LuaJIT in LuaTeX will likely be dropped in the future, because LuaJIT is frozen at Lua 5.1 (with some 5.2 extensions) and it becomes increasingly painful to write generic Lua code which works on both the latest PUC-Rio Lua and LuaJIT. This issue has become worse with the introduction of integers and the integer division operator
// because it breaks the existing behaviour of numerical calculations. Try for example
with LuaJIT and Lua 5.3.