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But if an alkyl group is attached to a benzene ring, we call the parent chain the benzene ring, no matter how many carbons the alkyl group has. Why? For example: heptylbenzene.

For another example: when 1-propenyl-benzene reacts with sulfuric acid we get 1-phenyl-1-propanol. We don't call this product 1-propanol-1-benzene.

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In your first example, heptylbenzene, the benzene ring is the parent structure for the preferred IUPAC name because the ring is always the parent structure if the ring and the chain have the same number of substituents that are cited as the principal characteristic group (by means of a suffix or a class name, or implied by a trivial name). The corresponding rule in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows.

P-52.2.8 Selection between a ring and a chain as parent hydride

Within the same heteroatom class and for the same number of characteristic groups cited as the principal characteristic group, a ring is always selected as the parent hydride to construct a preferred IUPAC name. In general nomenclature, a ring or a chain can be the parent hydride (see P-44.1.2.2).

In heptylbenzene, neither the benzene ring nor the heptane chain has any substituents that could be cited as the principal characteristic group. Therefore, the ring is selected as the parent structure.

In 1-phenylpropan-1-ol, however, the propane chain has an $\ce{-OH}$ group, which is the principal characteristic group of this compound and expressed as suffix (‘-ol’). Thus, Rule P-52.2.8 cannot be used. If the ring and the chain have different numbers of substituents corresponding to the principal characteristic group, another rule comes first, which stipulates that the senior parent structure is the one that has the maximum number of substituents cited as the principal characteristic group.

P-44.1 SENIORITY ORDER FOR PARENT STRUCTURES

When there is a choice, the senior parent structure is chosen by applying the following criteria, in order, until a decision is reached. These criteria must always be applied before those applicable to rings and ring systems (see P-44.2) and to chains (see P-44.3). Then criteria applicable to both chains and rings or ring systems given in P-44.4 are considered.

P-44.1.1 The senior parent structure has the maximum number of substituents corresponding to the principal characteristic group (suffix) or senior parent hydride in accord with the seniority of classes (P-41) and the seniority of suffixes (P-43).

Therefore, the propane chain is the parent structure in 1-phenylpropan-1-ol.

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In this case the -OH group attached to the benzene ring is a fuctional group of higher priority than the HCs, as such it will complete change the general property of the benzene(HC) to that of alcohols. I think this is the reason behind this.

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To the best of my knowledge, I think it is because the hydroxyl group (-OH) when connected to the benzene ring overrides the basic chemical properties of the benzene, displaying it's own more prominently. Thus it takes the priority in naming. But in the case of alkyl groups, they don't bring about much change to the basic properties of benzene and the chemical properties of the benzene remain more prominent. Thus benzene takes the priority in naming. However I believe this may change if the alkyl chain is relatively large and overrides the benzene's behavior in the molecule.
I haven't cross checked anywhere so I'm not entirely sure but that's what I think makes the most sense. Hope it helps.

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