Of course they don't believe it!
Democrats do a shit job at celebrating their victories and communicating with average Joes.
Also this asinine "liberal guilt" prevents anyone from the Left to ever enjoy anything for fear that there might be one person out there who isn't doing so well and it would hUrTiNg tHeIr fEeLiNgS. It is absolutely exhausting dealing with these people who don't want to accept that unemployment is at an incredibly low number, interest rates are still at historic lows, wage growth is at 40 year record, inflation is dropping, and the economy is indeed booming. Times are indeed good, regardless of what the vocal naysayers tell people.
Then mix in an endless stream of foreign agent trolls who are purposefully trying to spread misinformation before the election. Whether they are Russian agents or right-wing conservatives masquerading as progressives, they are trying to bring people down because apathy is one of the best tools that Republicans have to keep people home on election day. Low voter turn-out usually helps the GOP.
Put all that together and it isn't surprising that Americans don't believe that the economy is strong.