When writing out synthetic reaction schemes, why are some reactions combined in one arrow?
I want to start off by saying that your example is a particularly bad one. Usually, the arrows in a scheme should at least state a list of reagents and steps. Instead, I would love to discuss this using scheme 7 of Zhang et al.’s Beilstein J. Org. Chem. article.[1] (Beilstein J. Org. Chem. is a open access journal, the full text should be accessible from the DOI link provided below.)

The main reason for combining reactions like this is brevity. For example, every practicing chemist will immediately understand that the homoallyl Grignard under 1. will attack one of the carbonyl functions, that triethylsilane will reduce an oxygen and that TBAF will remove the silyl protecting group on the hydroxy function. None of these steps really warrant any discussion and most people would be able to draw out the missing structures in a few minutes. Likewise, I am not entirely sure why they did not include the preceeding protection into that same reaction scheme because there too there is noting interesting going on.

In more advanced synthetic schemes, the steps that are shortened and written together on a single arrow are often typical name reactions or very easily performed ones:


protections and deprotections
oxidations of alcohols to aldehydes with standard reagents
reductions with DIBAL, LAH or $\ce{NaBH4}$
ozonolyses
double bond isomerisations
simple Wittig/HWE reactions
etc.


They are typically chosen in such a way that the path is obvious and that the structure on the end of the arrow can be drawn up quickly by going through the steps. They will practically never include key steps or steps whose stereochemical outcome is uncommon. Cross-couplings of more elaborate structures or lesser known name reactions are also less common.

However, there is no rule as to what can be combined and what cannot. If anything, the reviewers might mark a scheme as too elaborate (i.e. can be shortened) or too brief (needs elaboration).

Reference:

[1]: J. Zhang, H.-K. Zhang, P.-Q. Huang, Beilstein J. Org. Chem. 2013, 9, 2358–2366. DOI: 10.3762/bjoc.9.271.