I've read many food and Chef forums as well as discussions about this and while there are several differences and reasons why a home cooked Steak never usually approaches a Steakhouse steak from one of the better Steakhouses.
- Steakhouses have huge High Temp Broilers that aren't available in homes but that are dedicated Steak cookers that cook at around or in excess of 900-1000 degrees.
- Much higher quality of meat, USDA Prime that is just not available in most supermarkets but sought out by the best Steakhouses, many times, from select butchers where the Steakhouse has an established relationship.
- The seasoning method does usually vary but usually involves abundant use of Kosher Salt usually freshly ground. There are other seasonings as well but the combination of Kosher Salt and High Temp marbling the best meat specially selected for it's fat content and marbling.
- The Steaks are specifically aged, most wet aged but the best are dry aged in special areas are provided/kept with conditions specifically maintained for perfect aging. The MSG and some other Chemicals are sometimes used to make up for this aging but usually don't really accomplish the same thing as wet/dry aging does to make it as tender.
- Ample use of Butter, some houses use special butters but like salt butter is not a bad thing (except to maybe the heath gestapo people).
There are other reasons why steak house steaks are said to be better but the above are some of the more commonly mentioned, from what I have read.