We didn't, we came up with "island hopping"; bypassed the heavily defended Japanese major bases like Rabaul & Truk, sometimes our intel was wrong and we invaded Peleliu or vanity got in the way and we invaded the Phillipines [instead of Formosa][sarcasm] Makes me wonder why we even bothered losing all those men defending and taking them back. [/sarcasm]
Even if we lost our entire navy at Midway, it would have not made a difference in Japan's ability to take Midway, they didn't have enough troops, [3,000 Army and Naval Landing troops were facing over 5,000 US troops supported by tanks] they didn't have any way for those troops to call in air support[the airstrike on Midway killed 5 soldiers and did not disable one artillery, or AA gun] and the Navy had no doctrine on shore bombardment. They detailed 4 cruisers to support the landing, none of them had any training in fire support; contrast that with the US NavyAlternate realities are fun to bandy about.
I imagine SOMEONE thought that unchecked, there was enough of a possibility. I don't think our defensive policy relied on the geography seeing how WE at one time took control of those islands.
There's more than one way to take territory. One way is to make the costs to keep it or get it too high. Didn't work out so well for Chamberlin but Japan might still have tried it under the right conditions. If Japan had won Midway and been able to hurt our Navy to the point of non-response (not knowing the existence of the bomb), they would have continued East as a means to keep us out of China. I can see a war weary US being done with Europe saying a few people in TERRITORIES not worth the lives to take and keep them, if we could get peace.
There was a lot of internal strife in the US towards the end of the war. Only so much political capital to expend. Midway is called "critical" for a reason I suppose.
The Japanese only had a few contested amphibious assaults during WW2,, only one of which was successful [ 2nd Wake, first Wake was a disaster] the others were disasters like Battle of the Points on Bataan when 2000 Japanese attacked a beach area defended by Phillippine police company, 300 US sailors and 150 US Army Air force ground personnel, all but 43 Japanese died.