For historical reasons.
The Int64 type was first introduced in Delphi4. And at the same time, the StrToInt64 function was added in addition to the existing StrToInt. Since overlapping functions (introduce in Delphi4 also) do not differ in the type of results, the names were chosen of course different. It is also worth noting that for 32-bit Intel processors, 32-bit integers and 10-bit floating-point numbers (extended) were native. Therefore, the functions StrToInt: Integer and StrToFloat:Extended were native, but StrToInt64: Int64 is optional. In later versions Delphi, TryStrTo... functions were added. With the preservation of names, of course.
The StrToQWord function was added in FPC because the QWord type did not fit into Int64. A similar StrToUInt64 function was also added, but later, in Delphi.